Add UMA stats for time in paint+layout, compositing, paint invalidation and paint.

third_party/WebKit/Source/platform/Histogram.h

Index: third_party/WebKit/Source/platform/Histogram.h
diff --git a/third_party/WebKit/Source/platform/Histogram.h b/third_party/WebKit/Source/platform/Histogram.h
index a88970454c9e2ff4dce33c2774f81081f6fcc363..542f321489be6d252cc4d67cda90b919020671b4 100644
--- a/third_party/WebKit/Source/platform/Histogram.h
+++ b/third_party/WebKit/Source/platform/Histogram.h
@@ -7,6 +7,7 @@
 
 #include "base/metrics/histogram_base.h"
 #include "platform/PlatformExport.h"
+#include "wtf/CurrentTime.h"
 #include <stdint.h>
 
 namespace base {
@@ -42,6 +43,33 @@ private:
     base::HistogramBase* m_histogram;
 };
 
+class PLATFORM_EXPORT ScopedUsHistogramTimer {
+public:
+    ScopedUsHistogramTimer(CustomCountHistogram& counter)
+    : m_startTime(WTF::monotonicallyIncreasingTime()),

[pdr., 2016/04/01 00:03:16]

Nit: indentation

[chrishtr, 2016/04/01 16:33:42]

Done

+        m_counter(counter) {}
+
+    ~ScopedUsHistogramTimer()
+    {
+        m_counter.count(WTF::monotonicallyIncreasingTime() - m_startTime);
+    }
+
+private:
+    double m_startTime;
+    CustomCountHistogram& m_counter;
+};
+
+// Use code like this to record time, in microseconds, to execute a block of code:
+//
+// {
+//     SCOPED_BLINK_UMA_HISTOGRAM_TIMER(myUmaStatName)
+//     RunMyCode();
+// }
+// This macro records all times between 0us and 10 seconds.

[wkorman, 2016/04/01 00:15:30]

In 95th pctile I believe we have seen for some metrics close to 10s, are we ok
seeing this max out in some cases?

[chrishtr, 2016/04/01 00:29:40]

I've been cribbing off of the implementation of SCOPED_UMA_HISTOGRAM_TIMER. I
think it bottoms out at UMA_HISTOGRAM_TIMES which uses 10s and 50 buckets. I'll
change to 50 buckets. There is also a _MEDIUM and _LONG variant implemented, but
I figured that is overkill.

[Alexei Svitkine (slow), 2016/04/01 15:26:12]

Yes, please change to 50 buckets. 100 bucket histograms are quite expensive and
often don't provide more insight than 50 bucket ones.

Note: The normal TIMES() macros in base/ are in ms, not μs, so they're not
necessarily a good point of comparison.

[chrishtr, 2016/04/01 16:33:41]

Done.

+#define SCOPED_BLINK_UMA_HISTOGRAM_TIMER(name) \
+DEFINE_STATIC_LOCAL(CustomCountHistogram, scopedUsCounter, (name, 0, 10000000, 100)); \
+ScopedUsHistogramTimer timer(scopedUsCounter);
+
 } // namespace blink
 
 #endif // Histogram_h