Add UMAs for SharedBuffer::lock()/unlock()

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 4b565c63a1b0cd3ac0201f7de7832f585e52b171..8a53f7b3a137bd1728edb3f8738387a44c1fdffb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3814,6 +3814,30 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Blink.SharedBuffer.FailedLock" enum="ResourceType">
+  <owner>hiroshige@chromium.org</owner>
+  <summary>
+    Number of failed SharedBuffer::lock() calls for each resource type.
+    https://crbug.com/603791
+  </summary>
+</histogram>
+
+<histogram name="Blink.SharedBuffer.SuccessfulLock" enum="ResourceType">
+  <owner>hiroshige@chromium.org</owner>
+  <summary>
+    Number of successful SharedBuffer::lock() calls for each resource type.
+    https://crbug.com/603791
+  </summary>
+</histogram>
+
+<histogram name="Blink.SharedBuffer.Unlock" enum="ResourceType">
+  <owner>hiroshige@chromium.org</owner>
+  <summary>
+    Number of SharedBuffer::unlock() calls for each resource type.
+    https://crbug.com/603791

[Ilya Sherman, 2016/07/11 19:28:33]

It's not clear to me, from looking at the bug, what the motivation for recording
these metrics is.  Could you please clarify?

[hiroshige, 2016/07/12 10:06:29]

Updated the CL description.

+  </summary>
+</histogram>
+
 <histogram name="Blink.StyleAndLayout.UpdateTime" units="microseconds">
   <owner>layout-dev@chromium.org</owner>
   <summary>
@@ -88287,8 +88311,9 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="8" label="Link prefetch"/>
   <int value="9" label="Link subresource"/>
   <int value="10" label="Text track"/>
-  <int value="11" label="Shader"/>
-  <int value="12" label="Import resource"/>
+  <int value="11" label="ImportResource"/>
+  <int value="12" label="Media"/>
+  <int value="13" label="Manifest"/>

[Ilya Sherman, 2016/07/11 19:28:33]

This is not a safe change -- the result is that bucket 11 will be labeled as
"ImportResource", but will in fact be a mix of data coming from "Shader" and
"ImportResource" (and bucket 12 will have a similar problem).  Do you know how
long ago the C++ enum was changed out from under the affected histogram(s)?  I'd
recommend renaming any affected histogram(s) so that the data is clearly
interpretable.  (Note that there will always be a long tail of users who are
"stuck" on older versions of Chrome, and who will continue to upload metrics. 
So even if the enum change happened a year ago, the data will continue to be
influenced by users who are uploading data using the older enum semantics.)

[hiroshige, 2016/07/12 10:06:29]

This enum is used by WebCore.ResourceFetcher.ResourceTypeUponCacheHit, but it
seems no longer used:
- No C++ code touching ResourceTypeUponCacheHit, and
- No UMA count in recent 28 days.
Probably WebCore.ResourceFetcher.ResourceTypeUponCacheHit is removed quite long
time ago and I expect it's safe to modify the values here (and deprecate
WebCore.ResourceFetcher.ResourceTypeUponCacheHit).

+clamy@ as the owner of the metric.

 </enum>
 
 <enum name="RestoredCachedStyleSheet" type="int">