Add UMA histogram that tracks time spent in background GC tasks.

This adds the following histograms to histograms.xml:
- V8.GCBackgroundMarking
- V8.GCBackgroundScavenger
- V8.GCBackgroundSweeping

Bug: chromium:792552
Change-Id: I15f3df49773b630620d6c67121c46ad7fff3e179
Reviewed-on: https://chromium-review.googlesource.com/819610Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523754}

tools/metrics/histograms/histograms.xml

@@ -92636,6 +92636,30 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="V8.GCBackgroundMarking" units="ms">
+  <owner>ulan@chromium.org</owner>
+  <summary>
+    Time spent in background tasks doing marking in one GC cycle. It is recorded
+    after each GC.
+  </summary>
+</histogram>
+
+<histogram name="V8.GCBackgroundScavenger" units="ms">
+  <owner>ulan@chromium.org</owner>
+  <summary>
+    Time spent in background tasks doing scavenging in one GC cycle. It is
+    recorded after each GC.
+  </summary>
+</histogram>
+
+<histogram name="V8.GCBackgroundSweeping" units="ms">
+  <owner>ulan@chromium.org</owner>
+  <summary>
+    Time spent in background tasks doing sweeping in one GC cycle. It is
+    recorded after each GC.
+  </summary>
+</histogram>
+
 <histogram name="V8.GCCompactor" units="ms">
   <owner>hpayer@chromium.org</owner>
   <summary>Time spent in mark-sweep phase of GC.</summary>