[oilpan] Report coalescing time to UMA.

Bug: 
Change-Id: I4f492623358849f6a296bfe7d6c85ffb52b85c73
Reviewed-on: https://chromium-review.googlesource.com/833936
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525290}

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

@@ -31,6 +31,7 @@
 #include "platform/heap/HeapPage.h"
 
 #include "base/trace_event/process_memory_dump.h"
+#include "platform/Histogram.h"
 #include "platform/MemoryCoordinator.h"
 #include "platform/bindings/ScriptForbiddenScope.h"
 #include "platform/heap/BlinkGCMemoryDumpProvider.h"
@@ -714,6 +715,8 @@ bool NormalPageArena::Coalesce() {
   DCHECK(!HasCurrentAllocationArea());
   TRACE_EVENT0("blink_gc", "BaseArena::coalesce");
 
+  double coalesce_start_time = WTF::CurrentTimeTicksInMilliseconds();
+
   // Rebuild free lists.
   free_list_.Clear();
   size_t freed_size = 0;
@@ -769,6 +772,14 @@ bool NormalPageArena::Coalesce() {
   GetThreadState()->Heap().HeapStats().DecreaseAllocatedObjectSize(freed_size);
   DCHECK_EQ(promptly_freed_size_, freed_size);
   promptly_freed_size_ = 0;
+
+  double coalesce_time =
+      WTF::CurrentTimeTicksInMilliseconds() - coalesce_start_time;
+  DEFINE_THREAD_SAFE_STATIC_LOCAL(
+      CustomCountHistogram, time_for_heap_coalesce_histogram,
+      ("BlinkGC.TimeForCoalesce", 1, 10 * 1000, 50));
+  time_for_heap_coalesce_histogram.Count(coalesce_time);
+
   return true;
 }