Validate subprocess histograms before and after merge.

All of the corrupted histograms are from subprocesses so adding checks
where subprocess histograms are merged into the browser (called by
SubprocessMetricsProvider::MergeHistogramDeltasFromAllocator).

Bug: 744734
Change-Id: Ie6593f2d2ad5fa2e957307b153248bc882174458
Reviewed-on: https://chromium-review.googlesource.com/598189Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491466}

base/metrics/persistent_histogram_allocator.cc

@@ -478,8 +478,16 @@ void PersistentHistogramAllocator::MergeHistogramDeltaToStatisticsRecorder(
     return;
   }
 
+  // TODO(bcwhite): Remove this when crbug/744734 is fixed.
+  histogram->ValidateHistogramContents(true, -1);
+  existing->ValidateHistogramContents(true, -2);
+
   // Merge the delta from the passed object to the one in the SR.
   existing->AddSamples(*histogram->SnapshotDelta());
+
+  // TODO(bcwhite): Remove this when crbug/744734 is fixed.
+  histogram->ValidateHistogramContents(true, -3);
+  existing->ValidateHistogramContents(true, -4);
 }
 
 void PersistentHistogramAllocator::MergeHistogramFinalDeltaToStatisticsRecorder(