reland 2: Switch back to field trial.

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 32064bce14ab6a29005954557c147bcb3460ab61..6324dbc56d936e45a5598db3568739a0e568b06a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -30036,6 +30036,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="OSX.SharedMemory.Mechanism" enum="OSXSharedMemoryMechanism">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    A histogram entry is emitted each time a base::SharedMemory object is
+    constructed. The value of the entry indicates the mechanism used to back the
+    shared memory region.
+  </summary>
+</histogram>
+
 <histogram name="OSX.SystemHotkeyMap.LoadSuccess" enum="BooleanSuccess">
   <owner>erikchen@chromium.org</owner>
   <summary>
@@ -67978,6 +67987,15 @@ To add a new entry, add it with any value and run test to compute valid value.
   </int>
 </enum>
 
+<enum name="OSXSharedMemoryMechanism" type="int">
+  <int value="0" label="POSIX">
+    The shared memory region is backed by a POSIX fd.
+  </int>
+  <int value="1" label="MACH">
+    The shared memory region is backed by a Mach memory object.
+  </int>
+</enum>
+
 <enum name="OtherPossibleUsernamesUsage" type="int">
   <int value="0" label="Nothing to Autofill"/>
   <int value="1" label="No other possible usernames"/>