Bump field trial allocator size to 64kb and report usage

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 2041dd4cb7ce57ca4dba3a1acfbd2c7c4ce3e738..9ceb93876d33df5190dfc524aee6dd5e805132d1 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -66653,6 +66653,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="UMA.FieldTrialAllocator.Size" units="bytes">
+  <obsolete>
+    Deprecated 11/2016 for UMA.FieldTrialAllocator.Used
+  </obsolete>
   <owner>lawrencewu@chromium.org</owner>
   <owner>bcwhite@chromium.org</owner>
   <owner>asvitkine@chromium.org</owner>
@@ -66951,6 +66954,27 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="UMA.PersistentAllocator.FieldTrialAllocator.Allocs"
+    units="bytes">
+  <owner>lawrencewu@chromium.org</owner>
+  <owner>bcwhite@chromium.org</owner>
+  <owner>asvitkine@chromium.org</owner>
+  <summary>
+    Size of objects allocated from persistent memory in the browser process for
+    field trials. Updated on each subprocess launch.
+  </summary>
+</histogram>
+
+<histogram name="UMA.PersistentAllocator.FieldTrialAllocator.UsedPct" units="%">
+  <owner>lawrencewu@chromium.org</owner>
+  <owner>bcwhite@chromium.org</owner>
+  <owner>asvitkine@chromium.org</owner>
+  <summary>
+    Percentage of persistent memory segment that has been allocated. This is
+    updated on each subprocess launch.
+  </summary>
+</histogram>
+
 <histogram name="UMA.PersistentAllocator.GpuMetrics.Allocs" units="bytes">
   <owner>bcwhite@chromium.org</owner>
   <owner>asvitkine@chromium.org</owner>