Add an uma stat to time Pending Tree activation

cc/base/histograms.h

 // Copyright 2015 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.
 
 #ifndef CC_BASE_HISTOGRAMS_H_
 #define CC_BASE_HISTOGRAMS_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_base.h"
@@ skipping 32 matching lines @@
 //       ScopedReticulateSplinesTimer,
 //       "Compositing.%s.ReticulateSplinesUs",
 //       "Compositing.%s.ReticulateSplinesPixelsPerMs");
 //
 //   // Inside a method.
 //   ScopedReticulateSplinesTimer timer;
 //   timer.AddArea(some_rect.size().GetArea());
 //
 #define DEFINE_SCOPED_UMA_HISTOGRAM_AREA_TIMER(class_name, time_histogram,  \
                                                area_histogram)              \
-  class class_name : public ::cc::ScopedUMAHistogramAreaTimerBase {         \
+  class class_name : public ScopedUMAHistogramAreaTimerBase {               \
    public:                                                                  \
     ~class_name();                                                          \
   };                                                                        \
   class_name::~class_name() {                                               \
     Sample time_sample;                                                     \
     Sample area_sample;                                                     \
-    const char* client_name = ::cc::GetClientNameForMetrics();              \
+    const char* client_name = GetClientNameForMetrics();                    \
     if (client_name && GetHistogramValues(&time_sample, &area_sample)) {    \
       /* GetClientNameForMetrics only returns one non-null value over */    \
       /* the lifetime of the process, so these histogram names are */       \
       /* runtime constant. */                                               \
       UMA_HISTOGRAM_COUNTS(base::StringPrintf(time_histogram, client_name), \
                            time_sample);                                    \
       UMA_HISTOGRAM_COUNTS(base::StringPrintf(area_histogram, client_name), \
                            area_sample);                                    \
     }                                                                       \
   }
 
+// Version of the above macro for cases which only care about time, not area.
+#define DEFINE_SCOPED_UMA_HISTOGRAM_TIMER(class_name, time_histogram)       \
+  class class_name : public ScopedUMAHistogramAreaTimerBase {               \
+   public:                                                                  \
+    ~class_name();                                                          \
+  };                                                                        \
+  class_name::~class_name() {                                               \
+    Sample time_sample;                                                     \
+    Sample area_sample;                                                     \
+    const char* client_name = GetClientNameForMetrics();                    \
+    if (client_name && GetHistogramValues(&time_sample, &area_sample)) {    \
+      DCHECK_EQ(0, area_sample);                                            \
+      /* GetClientNameForMetrics only returns one non-null value over */    \
+      /* the lifetime of the process, so these histogram names are */       \
+      /* runtime constant. */                                               \
+      UMA_HISTOGRAM_COUNTS(base::StringPrintf(time_histogram, client_name), \
+                           time_sample);                                    \
+    }                                                                       \
+  }
+
 class CC_EXPORT ScopedUMAHistogramAreaTimerBase {
  public:
   void AddArea(const base::CheckedNumeric<int>& area) { area_ += area; }
   void SetArea(const base::CheckedNumeric<int>& area) { area_ = area; }
 
  protected:
   using Sample = base::HistogramBase::Sample;
 
   ScopedUMAHistogramAreaTimerBase();
   ~ScopedUMAHistogramAreaTimerBase();
@@ skipping 11 matching lines @@
   base::ElapsedTimer timer_;
   base::CheckedNumeric<int> area_;
 
   friend class ScopedUMAHistogramAreaTimerBaseTest;
   DISALLOW_COPY_AND_ASSIGN(ScopedUMAHistogramAreaTimerBase);
 };
 
 }  // namespace cc
 
 #endif  // CC_BASE_HISTOGRAMS_H_