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

third_party/WebKit/Source/platform/HistogramTest.cpp

Index: third_party/WebKit/Source/platform/HistogramTest.cpp
diff --git a/third_party/WebKit/Source/platform/HistogramTest.cpp b/third_party/WebKit/Source/platform/HistogramTest.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..8b575b6c8ea741ea81b13d0dc28bd2bbdf2a218f
--- /dev/null
+++ b/third_party/WebKit/Source/platform/HistogramTest.cpp
@@ -0,0 +1,62 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "platform/Histogram.h"
+
+#include "base/metrics/histogram_samples.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace blink {
+
+class ScopedUsHistogramTimerTest : public ::testing::Test {
+public:
+    static void advanceClock(double microseconds)
+    {
+        s_timeElapsed += microseconds;
+    }
+
+protected:
+    static double returnMockTime()
+    {
+        return s_timeElapsed;
+    }
+
+    virtual void SetUp()
+    {
+        s_timeElapsed = 0.0;
+        m_originalTimeFunction = setTimeFunctionsForTesting(returnMockTime);
+    }
+
+    virtual void TearDown()
+    {
+        setTimeFunctionsForTesting(m_originalTimeFunction);
+    }
+
+private:
+    static double s_timeElapsed;
+    TimeFunction m_originalTimeFunction;
+};
+
+double ScopedUsHistogramTimerTest::s_timeElapsed;
+
+class TestCustomCountHistogram : public CustomCountHistogram {
+public:
+    TestCustomCountHistogram(const char* name, base::HistogramBase::Sample min, base::HistogramBase::Sample max, int32_t bucketCount)
+    : CustomCountHistogram(name, min, max, bucketCount) {}
+
+    base::HistogramBase* histogram() { return m_histogram; }
+};
+
+TEST_F(ScopedUsHistogramTimerTest, Basic)
+{
+    TestCustomCountHistogram scopedUsCounter("test", 0, 10000000, 50);
+    {
+        ScopedUsHistogramTimer timer(scopedUsCounter);
+        advanceClock(0.5);
+    }
+    // 0.5s == 500000us
+    EXPECT_EQ(500000, scopedUsCounter.histogram()->SnapshotSamples()->sum());
+}
+
+} // namespace blink