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..05879a4abdfb370835fff2c44828726c5b9edd95 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 {
@@ -23,7 +24,6 @@ public:
 protected:
     explicit CustomCountHistogram(base::HistogramBase*);
 
-private:
     base::HistogramBase* m_histogram;
 };
 
@@ -42,6 +42,36 @@ private:
     base::HistogramBase* m_histogram;
 };
 
+
+
+class PLATFORM_EXPORT ScopedUsHistogramTimer {
+public:
+    ScopedUsHistogramTimer(CustomCountHistogram& counter)
+        : m_startTime(WTF::monotonicallyIncreasingTime() * base::Time::kMicrosecondsPerSecond),

[Alexei Svitkine (slow), 2016/04/01 16:39:04]

Nit: Why not just do the multiplication on the delta in the end and keep the
intermediate as the base unit? This way, you only need to multiply once.

[chrishtr, 2016/04/01 16:43:44]

Done. Good idea.

+        m_counter(counter) {}
+
+    ~ScopedUsHistogramTimer()
+    {
+        m_counter.count(WTF::monotonicallyIncreasingTime() * base::Time::kMicrosecondsPerSecond - m_startTime);
+    }
+
+private:
+    // In microseconds.
+    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.

[Alexei Svitkine (slow), 2016/04/01 16:39:04]

Expand comment to mention that this macro shouldn't be changed without renaming
the metrics using these.

[chrishtr, 2016/04/01 16:43:44]

Done.

+#define SCOPED_BLINK_UMA_HISTOGRAM_TIMER(name) \
+DEFINE_STATIC_LOCAL(CustomCountHistogram, scopedUsCounter, (name, 0, 10000000, 50)); \
+ScopedUsHistogramTimer timer(scopedUsCounter);
+
 } // namespace blink
 
 #endif // Histogram_h