Support saving browser metrics to disk and reading them during next run.

base/metrics/histogram_unittest.cc

Index: base/metrics/histogram_unittest.cc
diff --git a/base/metrics/histogram_unittest.cc b/base/metrics/histogram_unittest.cc
index 58b72480018591f72679a6e1ff036727f25ada2c..e839cbcff3cd7c7c1b522bb9b6a76b856acb21f1 100644
--- a/base/metrics/histogram_unittest.cc
+++ b/base/metrics/histogram_unittest.cc
@@ -147,7 +147,7 @@ TEST_P(HistogramTest, NameMatchTest) {
   EXPECT_EQ(2, samples->GetCount(10));
 }
 
-// Check that delta calculations work correct.
+// Check that delta calculations work correctly.
 TEST_P(HistogramTest, DeltaTest) {
   HistogramBase* histogram =
       Histogram::FactoryGet("DeltaHistogram", 1, 64, 8,
@@ -176,6 +176,23 @@ TEST_P(HistogramTest, DeltaTest) {
   EXPECT_EQ(0, samples->TotalCount());
 }
 
+// Check that final-delta calculations work correctly.
+TEST_P(HistogramTest, FinalDeltaTest) {
+  HistogramBase* histogram =
+      Histogram::FactoryGet("FinalDeltaHistogram", 1, 64, 8,
+                            HistogramBase::kNoFlags);
+  histogram->Add(1);
+  histogram->Add(10);
+  histogram->Add(50);
+
+  std::unique_ptr<HistogramSamples> samples = histogram->SnapshotFinalDelta();

[Ilya Sherman, 2016/05/04 06:48:07]

Please include an additional test where the code first calls SnapshotDelta(). 
It's important to test both the codepath where there was no delta previously
recorded, and where there was one.

[bcwhite, 2016/05/04 14:12:50]

Done.

+  EXPECT_EQ(3, samples->TotalCount());
+  EXPECT_EQ(1, samples->GetCount(1));
+  EXPECT_EQ(1, samples->GetCount(10));
+  EXPECT_EQ(1, samples->GetCount(50));
+  EXPECT_EQ(samples->TotalCount(), samples->redundant_count());
+}
+
 TEST_P(HistogramTest, ExponentialRangesTest) {
   // Check that we got a nice exponential when there was enough room.
   BucketRanges ranges(9);