Add to RenderWidget::QueueMessage() comment.

TBR=piman@chromium.org

Change-Id: I8b7e62b44c596e95f14a4644c27bc4e99d883a5e
Reviewed-on: https://chromium-review.googlesource.com/c/1432293
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626258}

content/renderer/render_widget.h

@@ -419,7 +419,12 @@ class CONTENT_EXPORT RenderWidget
 
   void SetHandlingInputEvent(bool handling_input_event);
 
-  // Delivers |message| together with compositor state change updates.
+  // Queues the IPC |message| to be sent to the browser, delaying sending until
+  // the next compositor frame submission. At that time they will be sent before
+  // any message from the compositor as part of submitting its frame. This is
+  // used for messages that need synchronization with the compositor, but in
+  // general you should use Send().
+  //
   // This mechanism is not a drop-in replacement for IPC: messages sent this way
   // will not be automatically available to BrowserMessageFilter, for example.
   // FIFO ordering is preserved between messages enqueued.