[Reland] Track the last imp-frame

This is a re-land of:
https://chromium-review.googlesource.com/c/chromium/src/+/2079134

The previous CL had a problem where a tracker may not being terminated.
This CL fixes that and tests are added.

PS#3 is the same as the original CL, so that the diffs can be easily
seen.

Bug: 1055844
Change-Id: I62cffba355980013db8fa392fbb66d0ea74749af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2122242Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756018}

cc/metrics/frame_sequence_tracker.h

@@ -227,6 +227,8 @@ class CC_EXPORT FrameSequenceTrackerCollection {
   CustomTrackerResults TakeCustomTrackerResults();
 
   FrameSequenceTracker* GetTrackerForTesting(FrameSequenceTrackerType type);
+  FrameSequenceTracker* GetRemovalTrackerForTesting(
+      FrameSequenceTrackerType type);
 
   void SetUkmManager(UkmManager* manager);
 
@@ -234,6 +236,8 @@ class CC_EXPORT FrameSequenceTrackerCollection {
   friend class FrameSequenceTrackerTest;
 
   void RecreateTrackers(const viz::BeginFrameArgs& args);
+  // Destroy the trackers that are ready to be terminated.
+  void DestroyTrackers();
 
   const bool is_single_threaded_;
   // The callsite can use the type to manipulate the tracker.
@@ -489,9 +493,11 @@ class CC_EXPORT FrameSequenceTracker {
     void Advance(base::TimeTicks new_timestamp);
   } trace_data_;
 
-#if DCHECK_IS_ON()
+  // True when an impl-impl is not ended. A tracker is ready for termination
+  // only when the last impl-frame is ended (ReportFrameEnd).
   bool is_inside_frame_ = false;
 
+#if DCHECK_IS_ON()
   // This stringstream represents a sequence of frame reporting activities on
   // the current tracker. Each letter can be one of the following:
   // {'B', 'N', 'b', 'n', 'S', 'P'}, where