Add Memory.Experimental.Renderer2.V8.AllocatedObjects to histograms.xml

This is a follow-up to https://chromium-review.googlesource.com/1101020

Bug: 852415
Change-Id: If80d21fcd23749f33d8ca95543e56c8b990ed231
Reviewed-on: https://chromium-review.googlesource.com/1112015Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571868}

tools/metrics/histograms/histograms.xml

@@ -122410,6 +122410,8 @@ uploading your change for review.
       label="Only counting memory used by Skia bitmap resource cache."/>
   <suffix name="Sync" label="Only counting memory used by Sync storage."/>
   <suffix name="V8" label="Only counting memory used by V8 javascript engine."/>
+  <suffix name="V8.AllocatedObjects"
+      label="Only counting objects allocated by V8 javascript engine."/>
   <affected-histogram name="Memory.Experimental.Browser2"/>
   <affected-histogram name="Memory.Experimental.Extension2"/>
   <affected-histogram name="Memory.Experimental.Gpu2"/>