Add UMAs about Blink GC metrics

Source/platform/heap/ThreadState.cpp

Index: Source/platform/heap/ThreadState.cpp
diff --git a/Source/platform/heap/ThreadState.cpp b/Source/platform/heap/ThreadState.cpp
index aaef9c1d8a73899115ae6a0840800578c7e63b0b..94d84b9b640d64f6c756430819e31835b3a80e92 100644
--- a/Source/platform/heap/ThreadState.cpp
+++ b/Source/platform/heap/ThreadState.cpp
@@ -35,6 +35,7 @@
 #include "platform/heap/AddressSanitizer.h"
 #include "platform/heap/Handle.h"
 #include "platform/heap/Heap.h"
+#include "public/platform/Platform.h"
 #include "wtf/ThreadingPrimitives.h"
 
 #if OS(WIN)
@@ -819,8 +820,11 @@ void ThreadState::performPendingSweep()
 
     TRACE_EVENT0("Blink", "ThreadState::performPendingSweep");
     const char* samplingState = TRACE_EVENT_GET_SAMPLING_STATE();
-    if (isMainThread())
+    double timeStamp = 0;
+    if (isMainThread()) {
         TRACE_EVENT_SET_SAMPLING_STATE("Blink", "BlinkGCSweeping");
+        timeStamp = WTF::currentTimeMS();

[Mads Ager (chromium), 2014/05/28 09:35:45]

Is histogram collection also not thread safe? It would have been nice if we
could have histograms for all sweeps (also the ones not on the main thread).

[haraken, 2014/05/28 09:40:15]

It's thread-safe.

At first, I thought that if we collect sweeping time from other threads
(workers, database threads etc), the histogram distribution might be biased by
those non-main threads which will allocate very little objects. So I thought it
might be better to limit the UMA to the main thread.

Either is fine with me.

[haraken, 2014/05/28 10:40:26]

Done.

+    }
 
     m_sweepInProgress = true;
     // Disallow allocation during weak processing.
@@ -837,8 +841,12 @@ void ThreadState::performPendingSweep()
     clearGCRequested();
     clearSweepRequested();
 
-    if (isMainThread())
+    if (isMainThread()) {
         TRACE_EVENT_SET_NONCONST_SAMPLING_STATE(samplingState);
+        if (blink::Platform::current()) {
+            blink::Platform::current()->histogramEnumeration("BlinkGC.PerformPendingSweep", WTF::currentTimeMS() - timeStamp, 30000);
+        }
+    }
 }
 
 void ThreadState::addInterruptor(Interruptor* interruptor)