[bfcache] Add an experiment to show frame tree structured reason

This CL adds a new experiment on DevTools to show the bfcache blocking
reasons in a frame tree strucuture. This will be enabled by default
once we have all the reasons reported (currently there's a bug where
some reasons are missing).

Bug: 1288158
Change-Id: Id778a8b7320ef7092e280297add40cdd2936a299
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3396918
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Wolfgang Beyer <wolfi@chromium.org>
Commit-Queue: Yuzu Saijo <yuzus@chromium.org>

front_end/core/host/UserMetrics.ts

@@ -583,7 +583,8 @@ export enum DevtoolsExperiments {
   'groupAndHideIssuesByKind' = 51,
   'cssTypeComponentLength' = 52,
   'preciseChanges' = 53,
-  'MaxValue' = 54,
+  'bfcacheDisplayTree' = 54,
+  'MaxValue' = 55,
 }
 /* eslint-enable @typescript-eslint/naming-convention */
 

front_end/entrypoints/main/MainImpl.ts

@@ -297,6 +297,10 @@ export class MainImpl {
         'keyboardShortcutEditor', 'Enable keyboard shortcut editor', true,
         'https://developer.chrome.com/blog/new-in-devtools-88/#keyboard-shortcuts');
 
+    // Back/forward cache
+    Root.Runtime.experiments.register(
+        'bfcacheDisplayTree', 'Show back/forward cache blocking reasons in the frame tree structure view');
+
     // Timeline
     Root.Runtime.experiments.register('timelineEventInitiators', 'Timeline: event initiators');
     Root.Runtime.experiments.register('timelineInvalidationTracking', 'Timeline: invalidation tracking', true);