Remove DCHECK in frame_sequence_tracker.cc to deflake tests

This DCHECK is getting hit flakily in:
PageLoadMetricsBrowserTest.UseCounterFeaturesInNonSecureMainFrame
PageLoadMetricsBrowserTestWithAutoupgradesDisabled.
    UseCounterUkmMixedContentFeaturesLogged

on the Mac 10.13 dbg bot. Remove it for now to fix the bot, and add a
TODO to re-enable it once the cause of the flakes has been found.

TBR=sadrul@chromium.org

No-Tree-Checks: true
No-Try: true
Bug: 1066455
Change-Id: Ia30ae1bc3160dcbba4ea95fb0fd999bc95881750
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129901
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755020}

cc/metrics/frame_sequence_tracker.cc

@@ -971,8 +971,10 @@ void FrameSequenceTracker::ReportFramePresented(
           // if the main frame reports no-damage later on.
           impl_frames_produced_while_expecting_main_ += impl_frames_produced;
         } else {
-          DCHECK_EQ(impl_frames_produced_while_expecting_main_, 0u)
-              << TRACKER_DCHECK_MSG;
+          // TODO(https://crbug.com/1066455): Determine why this DCHECK is
+          // causing PageLoadMetricsBrowserTests to flake, and re-enable.
+          // DCHECK_EQ(impl_frames_produced_while_expecting_main_, 0u)
+          //    << TRACKER_DCHECK_MSG;
           aggregated_throughput().frames_produced += impl_frames_produced;
           impl_frames_produced_while_expecting_main_ = 0;
         }