Oilpan: Add reason arg to IncrementalMarkingStart trace event

Add reason arg to IncrementalMarkingStart trace event so we can see what trigger the incremental marking GC.

Bug: 757440
Change-Id: Ibecb88c157dec70e44287a146f93cdf7e5ef6211
Reviewed-on: https://chromium-review.googlesource.com/1124342Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572452}

third_party/blink/renderer/platform/heap/thread_state.cc

@@ -1451,7 +1451,8 @@ void ThreadState::IncrementalMarkingStart(BlinkGC::GCReason reason) {
   {
     ThreadHeapStatsCollector::Scope stats_scope(
         Heap().stats_collector(),
-        ThreadHeapStatsCollector::kIncrementalMarkingStartMarking);
+        ThreadHeapStatsCollector::kIncrementalMarkingStartMarking, "reason",
+        GcReasonString(reason));
     AtomicPauseScope atomic_pause_scope(this);
     next_incremental_marking_step_duration_ =
         kDefaultIncrementalMarkingStepDuration;