Add UMA metrics for root scroller intervention to track forcing passive breakage.

third_party/WebKit/Source/platform/Histogram.cpp

Index: third_party/WebKit/Source/platform/Histogram.cpp
diff --git a/third_party/WebKit/Source/platform/Histogram.cpp b/third_party/WebKit/Source/platform/Histogram.cpp
index cdda2dca45740c1d8e3b94f111baa2a32ed9f269..549d53beaceb61b8b16fe1c6c26c8d791988f448 100644
--- a/third_party/WebKit/Source/platform/Histogram.cpp
+++ b/third_party/WebKit/Source/platform/Histogram.cpp
@@ -6,6 +6,7 @@
 
 #include "base/metrics/histogram.h"
 #include "base/metrics/sparse_histogram.h"
+#include "base/metrics/statistics_recorder.h"
 
 namespace blink {
 
@@ -34,6 +35,19 @@ EnumerationHistogram::EnumerationHistogram(const char* name, base::HistogramBase
 {
 }
 
+bool EnumerationHistogram::GetHistogramCount(const char* name, base::HistogramBase::Sample bucket, int32_t* count)

[Rick Byers, 2016/07/11 14:41:52]

Probably want someone from uma-team to take a look at the testing strategy here.
 Does this mirror what's already done for histogram testing outside blink?  Eg.
I'm wondering if we should be clear that these functions are intended just for
testing.

[dtapuska, 2016/07/11 15:46:05]

isherman@ can you comment on this? (It is pretty much the same as
HistogramTester class.

[Ilya Sherman, 2016/07/11 18:52:31]

Yeah, the HistogramTester class [1] is what's used elsewhere in Chromium for
testing histograms.  It would be best if we could reuse that implementation. 
Note that the HistogramTester class does some work to initialize the
StatisticsRecorded, and ensure that each test sees deltas that are only based on
the code under test -- it essentially works as a scoped object.  I'm a little
concerned that the approach you have currently implemented will allow
dependencies across tests to form, because the metrics system is basically a
shared global object, and I don't see any code here that compensates for that.

[1] https://cs.chromium.org/chromium/src/base/test/histogram_tester.h

+{
+    base::HistogramBase* histogram = base::StatisticsRecorder::FindHistogram(name);
+    if (!histogram)
+        return false;
+
+    std::unique_ptr<base::HistogramSamples> samples(
+        histogram->SnapshotSamples());
+
+    *count = samples->GetCount(bucket);
+    return true;
+}
+
 SparseHistogram::SparseHistogram(const char* name)
 {
     m_histogram = base::SparseHistogram::FactoryGet(name, base::HistogramBase::kUmaTargetedHistogramFlag);