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 fc5adae2e4e25202528f57d07bcb83fd85d19765..1df300edcf351a4aa81f8e8260241dfdfa1ea483 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
+  </summary>
+</histogram>
+
 <histogram name="Blink.StyleAndLayout.UpdateTime" units="microseconds">
   <owner>layout-dev@chromium.org</owner>
   <summary>
@@ -88349,8 +88373,9 @@ 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, 2016/07/12 21:23:17]

If the only existing histogram that used this enum is no longer recorded, then
this change is mostly safe.  In that case, though, please mark that histogram as
<obsolete>, and add a note indicating that the enum labels for it are not
entirely reliable.

[hiroshige, 2016/07/15 07:47:05]

Done.

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