viz: Improve Comments Quality for Overlay Code

Follow up on previous CL by improving comments quality.

R=rjkroege

Bug: 979788
Change-Id: I110404464582bdf6490100fb601dda39ee0aca73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023481
Commit-Queue: weiliangc <weiliangc@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736189}

components/viz/service/display/overlay_processor_android.cc

@@ -109,7 +109,7 @@ void OverlayProcessorAndroid::ScheduleOverlays(
 
 void OverlayProcessorAndroid::OverlayPresentationComplete() {
   // This is a signal from Display::DidReceiveSwapBuffersAck. We use this to
-  // help clear locks on resources from old frame.
+  // help clear locks on resources from the old frame.
   committed_overlay_locks_.clear();
   std::swap(committed_overlay_locks_, pending_overlay_locks_.front());
   pending_overlay_locks_.pop_front();

components/viz/service/display/overlay_processor_android.h

@@ -91,7 +91,9 @@ class VIZ_SERVICE_EXPORT OverlayProcessorAndroid
   using OverlayResourceLock =
       DisplayResourceProvider::ScopedReadLockSharedImage;
 
-  // Locks for overlays are pending for OverlayPresentationComplete.
+  // Keep locks on overlay resources to keep them alive. Since we don't have
+  // an exact signal on when the overlays are done presenting, use
+  // OverlayPresentationComplete as a signal to clear locks from the older frames.
   base::circular_deque<std::vector<OverlayResourceLock>> pending_overlay_locks_;
   // Locks for overlays have been committed. |pending_overlay_locks_| will
   // be moved to |committed_overlay_locks_| after OverlayPresentationComplete.

components/viz/service/display/overlay_processor_interface.h

@@ -134,18 +134,20 @@ class VIZ_SERVICE_EXPORT OverlayProcessorInterface {
 
   // Before the overlay refactor to use OverlayProcessorOnGpu, overlay
   // candidates are stored inside DirectRenderer. Those overlay candidates are
-  // later sent over to the GPU thread by GLRenderer or SkiaRenderer. This is a
-  // helper function to take these overlay candidates inside overlay processor
-  // to avoid sending over DirectRenderer implementation. This is overridden by
-  // each platform that is ready to send overlay candidates inside overlay
-  // processo. Must be called before ScheduleOverlays().
+  // later sent over to the GPU thread by GLRenderer or SkiaRenderer. This
+  // helper function will be called by DirectRenderer to take these overlay
+  // candidates inside overlay processor to avoid sending over DirectRenderer
+  // implementation. This is overridden by each platform that is ready to send
+  // overlay candidates inside |OverlayProcessor|. Must be called before
+  // ScheduleOverlays().
   virtual void TakeOverlayCandidates(CandidateList* candidate_list) {}
 
   // TODO(weiliangc): Make it pure virtual after it is implemented by every
   // subclass.
   virtual void ScheduleOverlays(
       DisplayResourceProvider* display_resource_provider);
-  // This is a signal from Display::DidReceiveSwapBuffersAck.
+  // This is a signal from Display::DidReceiveSwapBuffersAck. This is used as
+  // approximate signale for when the overlays are presented.
   virtual void OverlayPresentationComplete();
 
   // These two functions are used by Android SurfaceControl.

components/viz/service/display/overlay_processor_on_gpu.cc

@@ -30,16 +30,16 @@ void OverlayProcessorOnGpu::ScheduleOverlays(
   for (auto& overlay : overlay_candidates) {
     auto shared_image_overlay =
         shared_image_representation_factory_->ProduceOverlay(overlay.mailbox);
-    // When display is re-opened, the first few frames might not have video
+    // When the display is re-opened, the first few frames might not have a video
     // resource ready. Possible investigation crbug.com/1023971.
     if (!shared_image_overlay)
       continue;
-    // In current implementation, the BeginReadAccess will ends up calling
+    // In the current implementation, the BeginReadAccess will end up calling
     // CodecImage::RenderToOverlay. Currently this code path is only used for
     // Android Classic video overlay, where update of the overlay plane is
-    // within media code. Since we are not actually passing an overlay plane to
-    // the display controller here, we are able to call EndReadAccess directly
-    // after BeginReadAccess.
+    // within the media code. Since we are not actually passing an overlay plane
+    // to the display controller here, we are able to call EndReadAccess
+    // directly after BeginReadAccess.
     shared_image_overlay->NotifyOverlayPromotion(
         true, ToNearestRect(overlay.display_rect));
     std::unique_ptr<gpu::SharedImageRepresentationOverlay::ScopedReadAccess>