Build fix for libstdc++

When building against libstdc++, the macro DCHECK_NE does not seem to
handle implicit conversion between std::unique_ptr and nullptr. This
patch fixes one such build error in compositor_frame_reporter.cc.

Bug: 957519
Change-Id: If1801dba777a90089a4c0585cec7e62de8006fff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595279Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Hans-Filip Elo <helo@vewd.com>
Cr-Commit-Position: refs/heads/master@{#838269}

cc/metrics/compositor_frame_reporter.cc

@@ -767,7 +767,7 @@ void CompositorFrameReporter::ReportCompositorLatencyHistogram(
 
 void CompositorFrameReporter::ReportEventLatencyHistograms() const {
   for (const auto& event_metrics : events_metrics_) {
-    DCHECK_NE(event_metrics, nullptr);
+    DCHECK(event_metrics);
     const std::string histogram_base_name =
         GetEventLatencyHistogramBaseName(*event_metrics);
     const int event_type_index = static_cast<int>(event_metrics->type());