[RPP] Cache message post events experiment check

So that we don't have to do the check on every node appended to thread
tracks, as it adds up to become expensive.

Bug: none
Change-Id: If2f6d053a9abeae22fdd46615356b55d08bb46a2
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/5956836
Commit-Queue: Nancy Li <[email protected]>
Commit-Queue: Andres Olivares <[email protected]>
Reviewed-by: Nancy Li <[email protected]>
Auto-Submit: Andres Olivares <[email protected]>

Author: and-oli

front_end/panels/timeline/CompatibilityTracksAppender.ts

@@ -32,6 +32,17 @@ export type HighlightedEntryInfo = {
   additionalElement?: HTMLElement,
 };
 
+let showPostMessageEvents: boolean|undefined;
+function isShowPostMessageEventsEnabled(): boolean {
+  // Everytime the experiment is toggled devtools is reloaded so the
+  // cache is updated automatically.
+  if (showPostMessageEvents === undefined) {
+    showPostMessageEvents =
+        Root.Runtime.experiments.isEnabled(Root.Runtime.ExperimentName.TIMELINE_SHOW_POST_MESSAGE_EVENTS);
+  }
+  return showPostMessageEvents;
+}
+
 export function entryIsVisibleInTimeline(
     entry: Trace.Types.Events.Event, parsedTrace?: Trace.Handlers.Types.ParsedTrace): boolean {
   if (parsedTrace && parsedTrace.Meta.traceIsGeneric) {
@@ -49,7 +60,7 @@ export function entryIsVisibleInTimeline(
     return true;
   }
 
-  if (Root.Runtime.experiments.isEnabled(Root.Runtime.ExperimentName.TIMELINE_SHOW_POST_MESSAGE_EVENTS)) {
+  if (isShowPostMessageEventsEnabled()) {
     if (Trace.Types.Events.isSchedulePostMessage(entry) || Trace.Types.Events.isHandlePostMessage(entry)) {
       return true;
     }