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 165273860e89d4b69598e4ecfdddd16d2d59bcb2..95c9eb0d8c7a52d20480ff758e1ce582ff3e94b0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -66637,6 +66637,9 @@ 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>
@@ -66646,6 +66649,26 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="UMA.FieldTrialAllocator.Used" units="bytes">
+  <owner>lawrencewu@chromium.org</owner>
+  <owner>bcwhite@chromium.org</owner>
+  <owner>asvitkine@chromium.org</owner>
+  <summary>
+    Total size of objects allocated from persistent memory in the browser
+    process for field trials. Updated on each subprocess launch.
+  </summary>
+</histogram>
+
+<histogram name="UMA.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.FieldTrialsEnabledBenchmarking" enum="BooleanUsage">
   <obsolete>
     Deprecated 2012. No longer tracked.