UMA: Add V8.GCMarkCompactor histogram

V8 CL: http://crrev.com/c/1256764

Bug: 843903
Change-Id: I347a507cb25ae86d911ca3863ae79efb57cf2f1d
Reviewed-on: https://chromium-review.googlesource.com/c/1286147
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600573}

tools/metrics/histograms/histograms.xml

@@ -114335,6 +114335,14 @@ uploading your change for review.
   <summary>Time spent in LowMemoryNotifications.</summary>
 </histogram>
 
+<histogram name="V8.GCMarkCompactor" units="ms">
+  <owner>mlippautz@chromium.org</owner>
+  <summary>
+    Sum of all durations of individual phases within one V8 mark-compact garbage
+    collection. Reported once per garbage collection.
+  </summary>
+</histogram>
+
 <histogram name="V8.GCMarkCompactReason" enum="GarbageCollectionReason">
   <owner>ulan@chromium.org</owner>
   <summary>Reason a mark-compact garbage collection was started in V8.</summary>