cc/metrics: Remove stray comment accidentally checked in.

BUG=none
R=xidachen@chromium.org

Change-Id: I821d77ecb5c79405ddbbb2a020f5a90214fef3ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028113Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736479}

cc/metrics/frame_sequence_tracker.cc

@@ -817,7 +817,6 @@ void FrameSequenceTracker::ReportMainFrameCausedNoDamage(
   // It is possible for |awaiting_main_response_sequence_| to be zero here if a
   // commit had already happened before (e.g. B(x)E(x)N(x)). So check that case
   // here.
-  // XXX: Add a DCHECK() at least to make sure that's what happened?
   if (awaiting_main_response_sequence_) {
     DCHECK_EQ(awaiting_main_response_sequence_, args.frame_id.sequence_number)
         << TRACKER_DCHECK_MSG;