Adding cache hit/miss histograms to DiskBasedCertCache.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index e35ae92e0c9f08a2a52273a11e3c56362403f57b..1c5911fdb00d13fa09a8a115695e0eb1df356ded 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3499,6 +3499,22 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="DiskBasedCertCache.DiskCacheHits">

[Ryan Sleevi, 2014/07/09 00:10:10]

Rather than describing in prose what true/false (really, 0/1) mean, you can use
enum="SomeEnumName" to describe what the values are.

See BooleanEnabled or BooleanSuccess for examples of this.

+  <owner>brandonsalmon@chromium.org</owner>
+  <summary>
+    Measure cache hits and misses when attempting to read a certificate from
+    disk using DiskBasedCertCache. True for hits, false for misses.

[Ryan Sleevi, 2014/07/09 00:10:10]

Measure whether a certificate was found cached on disk by the DiskBasedCertCache
?

+  </summary>
+</histogram>
+
+<histogram name="DiskBasedCertCache.MemCacheHits">
+  <owner>brandonsalmon@chromium.org</owner>
+  <summary>
+    Measures cache hits and misses for DiskBasedCertCache's in-memory MRU cache.
+    True for hits, false for misses.

[Ryan Sleevi, 2014/07/09 00:10:10]

Measure whether a certificate was found in net::DiskBasedCertCache's in-memory
MRU cache ?

+  </summary>
+</histogram>
+
 <histogram name="DiskCache.0.FilesAge" units="hours">
   <owner>rvargas@chromium.org</owner>
   <summary>The age of the cache's files (wall time).</summary>