Record UMA stats for subframes history navigations.

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 6b38741b1c42684cbc4efe6a3a8ca52982f8d5a9..3da6fbed3ab925b332579d759b631e6c0271cb0c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -55776,6 +55776,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="SessionRestore.RestoredSubframeURL" enum="BooleanRestoredURL">
+  <owner>creis@chromium.org</owner>
+  <summary>
+    How often history navigations in subframes restore a different URL than the
+    frame's default src URL.  This indicates how much users rely on subframe
+    session history items.
+  </summary>
+</histogram>
+
+<histogram name="SessionRestore.RestoreSubframeFramePathLength" units="bytes">
+  <owner>creis@chromium.org</owner>
+  <summary>
+    Records the length of unique names that include frame paths, for subframes
+    that are restoring a different URL than the frame's default src URL during a
+    history navigation.  Large values here would indicate a possible challenge
+    for the plan to truncate frame unique names (to save memory).
+  </summary>
+</histogram>
+
 <histogram name="SessionRestore.SaveLongPeriod">
   <owner>jeremy@chromium.org</owner>
   <owner>sky@chromium.org</owner>
@@ -73190,6 +73209,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Reset"/>
 </enum>
 
+<enum name="BooleanRestoredURL" type="int">
+  <int value="0" label="Default frame src URL"/>
+  <int value="1" label="Different URL"/>
+</enum>
+
 <enum name="BooleanReused" type="int">
   <int value="0" label="Not Reused"/>
   <int value="1" label="Reused"/>