Use SharedPersistentMemoryAllocator to share field trial state

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 59248e4829144c9203000e57ca49771257d84e75..dbfa93bea3744850af555349afba50fd437fd7bd 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -65616,6 +65616,27 @@ 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, before padding, of objects allocated from persistent memory in the
+    browser process for field trials. This is updated with every allocation.
+  </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 only once per reporting cycle.
+  </summary>
+</histogram>
+
 <histogram name="UMA.PersistentAllocator.GpuMetrics.Allocs" units="bytes">
   <owner>bcwhite@chromium.org</owner>
   <owner>asvitkine@chromium.org</owner>