PlzNavigate: Add metrics to understand bf navigations PLT regression

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 831235bd30b115df43e9fca3984b9e5084a96642..d8ea5e4f0a31717603ea484dbd8bef7e662fbd60 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -33257,6 +33257,40 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Navigation.BackForward.CacheFlags" enum="NavigationCacheEnum">
+  <owner>clamy@chromium.org</owner>
+  <summary>The cache flags set on back-forward navigations.</summary>
+</histogram>
+
+<histogram name="Navigation.BackForward.CommitTime" units="ms">
+  <owner>clamy@chromium.org</owner>
+  <summary>
+    The time needed to commit a back forward-navigation once it is ready to
+    commit. This is the time between ReadyToCommit and DidFinishNavigation (for
+    a navigation that commits).
+  </summary>
+</histogram>
+
+<histogram name="Navigation.BackForward.IsSameProcess" enum="BooleanEnabled">

[Ilya Sherman, 2017/06/13 21:38:00]

Please add a custom enum to enums.xml, with labels appropriate specifically for
this histogram.

[clamy, 2017/06/14 13:26:50]

Done.

+  <owner>clamy@chromium.org</owner>
+  <summary>
+    Whether the back-forward navigation lead to a change of process or not.
+  </summary>
+</histogram>
+
+<histogram name="Navigation.BackForward.TimeToReadyToCommit" units="ms">
+  <owner>clamy@chromium.org</owner>
+  <summary>
+    The time delta between the start of a back-forward navigation and the time
+    it was ready to commit.
+  </summary>
+</histogram>
+
+<histogram name="Navigation.BackForward.WasCached" enum="BooleanEnabled">

[Ilya Sherman, 2017/06/13 21:38:00]

Ditto.

[clamy, 2017/06/14 13:26:50]

Done.

+  <owner>clamy@chromium.org</owner>
+  <summary>Whether the back-forward navigation was served from cache.</summary>
+</histogram>
+
 <histogram name="Navigation.ClearSiteData.Duration" units="ms">
   <owner>msramek@chromium.org</owner>
   <summary>