paint-time: Remove a DCHECK for timestamp comparison.

The swap timestamp comes from the gpu process, and so it can not be
easily compared to the paint timestamp, which comes from the renderer
process. So remove the DCHECK that expects an ordering between the
two. A similar check was removed earlier in crrev.com/c/1063323

BUG=811961

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ia80306f78ec82e875ec6ebe7709cf4b445a40768
Reviewed-on: https://chromium-review.googlesource.com/1102083
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567716}

third_party/blink/renderer/core/paint/first_meaningful_paint_detector.cc

@@ -314,7 +314,6 @@ void FirstMeaningfulPaintDetector::SetFirstMeaningfulPaint(
     TimeTicks stamp,
     TimeTicks swap_stamp) {
   DCHECK(paint_timing_->FirstMeaningfulPaint().is_null());
-  DCHECK_GE(swap_stamp, stamp);
   DCHECK(!swap_stamp.is_null());
   DCHECK(network2_quiet_reached_);