[RPP] Don't make excessive copies of trace during export

- In Base64.ts `encode`, we were using `.split`. That creates copies of
  the very large string. Instead, use `.slice` to leverage V8's sliced
  strings optimization.
- In TimelinePanel.ts `innerSaveToFile`, we retained large copies of the
  trace data as Blob/BlobPart throughout the entire lifetime of the
  function. Instead, clear the variables when done to free them sooner.

I suspect these issues greatly raised the amount of memory needed to
export a trace, and are direct causes of the referenced OOM bug.

Bug: 450046606
Change-Id: I05a2ad3e30e16bd69a49501b56335b6dfde985d3
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7453170
Auto-Submit: Connor Clark <cjamcl@chromium.org>
Commit-Queue: Connor Clark <cjamcl@chromium.org>
Reviewed-by: Paul Irish <paulirish@chromium.org>
Commit-Queue: Paul Irish <paulirish@chromium.org>

Author: Connor Clark

front_end/core/common/Base64.ts

@@ -37,8 +37,15 @@ export function encode(input: BlobPart): Promise<string> {
     const reader = new FileReader();
     reader.onerror = () => reject(new Error('failed to convert to base64'));
     reader.onload = () => {
+      // This string can be very large, so take care to not double memory. `split`
+      // was used here before, which always results in new strings in V8. By using
+      // slice instead, we leverage the sliced string optimization in V8 and avoid
+      // doubling the memory requirement (even if temporarily: that is a potential
+      // source of OOM crashes given large enough input, such as is common with
+      // Performance traces).
       const blobAsUrl = reader.result as string;
-      const [, base64] = blobAsUrl.split(',', 2);
+      const index = blobAsUrl.indexOf(',');
+      const base64 = blobAsUrl.slice(index + 1);
       resolve(base64);
     };
 

front_end/panels/timeline/TimelinePanel.ts

@@ -1540,6 +1540,7 @@ export class TimelinePanel extends Common.ObjectWrapper.eventMixin<EventTypes, t
     }
 
     let blob = new Blob(blobParts, {type: 'application/json'});
+    blobParts.length = 0;  // Don't retain this large object for the remaining lifetime of this function.
 
     if (config.shouldCompress) {
       this.statusDialog.updateStatus(i18nString(UIStrings.compressingTraceForDownload));
@@ -1560,6 +1561,8 @@ export class TimelinePanel extends Common.ObjectWrapper.eventMixin<EventTypes, t
       //  blobParts.join('') === (await gzBlob.arrayBuffer().then(bytes => Common.Gzip.arrayBufferToString(bytes)))
     }
 
+    const blobType = blob.type;  // blob may be reassigned later.
+
     // In some cases Base64.encode() can return undefined; see crbug.com/436482118 for details.
     // TODO(crbug.com/436482118): understand this edge case and fix the Base64.encode method to not just return undefined.
     let bytesAsB64: string|null = null;
@@ -1569,11 +1572,14 @@ export class TimelinePanel extends Common.ObjectWrapper.eventMixin<EventTypes, t
       this.statusDialog.updateStatus(i18nString(UIStrings.encodingTraceForDownload));
       this.statusDialog.updateProgressBar(i18nString(UIStrings.encodingTraceForDownload), 100);
       bytesAsB64 = await Common.Base64.encode(blob);
+      if (bytesAsB64.length) {
+        blob = new Blob();  // Don't retain this large object for the remaining lifetime of this function.
+      }
     } catch {
     }
 
     if (bytesAsB64?.length) {
-      const contentData = new TextUtils.ContentData.ContentData(bytesAsB64, /* isBase64=*/ true, blob.type);
+      const contentData = new TextUtils.ContentData.ContentData(bytesAsB64, /* isBase64=*/ true, blobType);
       await Workspace.FileManager.FileManager.instance().save(fileName, contentData, /* forceSaveAs=*/ true);
       Workspace.FileManager.FileManager.instance().close(fileName);
     } else {