[oilpan] Report total garbage collection time to UMA.

Bug: 
Change-Id: Ia4a87a4fa6e98902bca8b3328399dcd4de79af3a
Reviewed-on: https://chromium-review.googlesource.com/833938
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525299}

third_party/WebKit/Source/platform/heap/ThreadState.cpp

@@ -1247,8 +1247,6 @@ void ThreadState::IncrementalMarkingFinalize() {
 void ThreadState::CollectGarbage(BlinkGC::StackState stack_state,
                                  BlinkGC::GCType gc_type,
                                  BlinkGC::GCReason reason) {
-  double start_time = WTF::CurrentTimeTicksInMilliseconds();
-
   // Nested garbage collection invocations are not supported.
   CHECK(!IsGCForbidden());
   // Garbage collection during sweeping is not supported. This can happen when
@@ -1256,6 +1254,9 @@ void ThreadState::CollectGarbage(BlinkGC::StackState stack_state,
   if (SweepForbidden())
     return;
 
+  double start_total_collect_garbage_time =
+      WTF::CurrentTimeTicksInMilliseconds();
+
   CompleteSweep();
 
   RUNTIME_CALL_TIMER_SCOPE_IF_ISOLATE_EXISTS(
@@ -1285,9 +1286,15 @@ void ThreadState::CollectGarbage(BlinkGC::StackState stack_state,
 
   PreSweep(gc_type);
 
+  double total_collect_garbage_time =
+      WTF::CurrentTimeTicksInMilliseconds() - start_total_collect_garbage_time;
+  DEFINE_THREAD_SAFE_STATIC_LOCAL(
+      CustomCountHistogram, time_for_total_collect_garbage_histogram,
+      ("BlinkGC.TimeForTotalCollectGarbage", 1, 10 * 1000, 50));
+  time_for_total_collect_garbage_histogram.Count(total_collect_garbage_time);
   VLOG(1) << "[state:" << this << "]"
           << " CollectGarbage: time: " << std::setprecision(2)
-          << (WTF::CurrentTimeTicksInMilliseconds() - start_time) << "ms"
+          << total_collect_garbage_time << "ms"
           << " stack: " << StackStateString(stack_state)
           << " type: " << GcTypeString(gc_type)
           << " reason: " << GcReasonString(reason);