Revert "[Memory] Unobserved isolates on panel hide"

This reverts commit 6d92651.

Reason for revert: Breaks the memory panel: Switching back and forth between the memory panel and every other panel will accumulate isolates in the list view.

Original change's description:
> [Memory] Unobserved isolates on panel hide
>
> This stops the `Runtime.getHeapUsage` getting called in the
> background.
> Mostly done to make it easier to debug DevTools on DevTools.
> But should be a small performance win as well.
>
> Bug: none
> Change-Id: Ief3e699436e61d3aa6917e5205c33e8f221c9d30
> Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7213820
> Auto-Submit: Nikolay Vitkov <nvitkov@chromium.org>
> Reviewed-by: Danil Somsikov <dsv@chromium.org>
> Commit-Queue: Nikolay Vitkov <nvitkov@chromium.org>

Bug: none
Change-Id: If2614db8d9254452b02f3b3b5ca57d9c34f2b6ac
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7513267
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>

Author: szuend

front_end/core/sdk/IsolateManager.ts

@@ -49,13 +49,6 @@ export class IsolateManager extends Common.ObjectWrapper.ObjectWrapper<EventType
     }
   }
 
-  unobserveIsolates(observer: Observer): void {
-    this.#observers.delete(observer);
-    if (!this.#observers.size) {
-      this.#pollId++;
-    }
-  }
-
   modelAdded(model: RuntimeModel): void {
     void this.#modelAdded(model);
   }

front_end/panels/profiler/IsolateSelector.ts

@@ -88,6 +88,7 @@ export class IsolateSelector extends UI.Widget.VBox implements UI.ListControl.Li
         this.totalTrendDiv, i18nString(UIStrings.totalPageJsHeapSizeChangeTrend, {PH1: trendIntervalMinutes}));
     UI.Tooltip.Tooltip.install(this.totalValueDiv, i18nString(UIStrings.totalPageJsHeapSizeAcrossAllVm));
 
+    SDK.IsolateManager.IsolateManager.instance().observeIsolates(this);
     SDK.TargetManager.TargetManager.instance().addEventListener(
         SDK.TargetManager.Events.NAME_CHANGED, this.targetChanged, this);
     SDK.TargetManager.TargetManager.instance().addEventListener(
@@ -96,7 +97,6 @@ export class IsolateSelector extends UI.Widget.VBox implements UI.ListControl.Li
 
   override wasShown(): void {
     super.wasShown();
-    SDK.IsolateManager.IsolateManager.instance().observeIsolates(this);
     SDK.IsolateManager.IsolateManager.instance().addEventListener(
         SDK.IsolateManager.Events.MEMORY_CHANGED, this.heapStatsChanged, this);
   }
@@ -105,7 +105,6 @@ export class IsolateSelector extends UI.Widget.VBox implements UI.ListControl.Li
     super.willHide();
     SDK.IsolateManager.IsolateManager.instance().removeEventListener(
         SDK.IsolateManager.Events.MEMORY_CHANGED, this.heapStatsChanged, this);
-    SDK.IsolateManager.IsolateManager.instance().unobserveIsolates(this);
   }
 
   isolateAdded(isolate: SDK.IsolateManager.Isolate): void {