Fix compositor thread name

Some benchmarks depend on fixed compositor thread name. Revert the name
back to the old one for compatibility.

TBR=haraken@chromium.org
BUG=823315

Change-Id: I07c324016280c33b240fe5f32e5c560ddd9250db
Reviewed-on: https://chromium-review.googlesource.com/969042Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Hector Dearman <hjd@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hector Dearman <hjd@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544110}

third_party/WebKit/Source/platform/WebThreadType.cpp

@@ -15,7 +15,8 @@ const char* GetNameForThreadType(WebThreadType thread_type) {
     case WebThreadType::kUnspecifiedWorkerThread:
       return "unspecified worker thread";
     case WebThreadType::kCompositorThread:
-      return "Compositor thread";
+      // Some benchmarks depend on this value.
+      return "Compositor";
     case WebThreadType::kDedicatedWorkerThread:
       return "DedicatedWorker thread";
     case WebThreadType::kSharedWorkerThread: