Add the Tab.TimeToReactivation metric to TabUsageRecorder

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 e97c34d19d4b26e81c1442e27f24cfa0229fa8ae..0dc0eda6aeccf3175fa385cd96ac5183f90105b0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -64072,6 +64072,10 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="Tab.Deactivation.Bookmarked" enum="Boolean">
+  <obsolete>
+    Deprecated 11/2016. No longer useful after finding out that it has no effect
+    on tab reactivation rates.
+  </obsolete>
   <owner>pmonette@chromium.org</owner>
   <summary>
     A tab was deactivated. Closing tabs are not included. This histogram also
@@ -64203,6 +64207,10 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="Tab.Reactivation.Bookmarked" enum="Boolean">
+  <obsolete>
+    Deprecated 11/2016. No longer useful after finding out that it has no effect
+    on tab reactivation rates.
+  </obsolete>
   <owner>pmonette@chromium.org</owner>
   <summary>
     A tab was reactivated after being hidden. This histogram also records if the
@@ -64394,6 +64402,23 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Tab.TimeToReactivation.Important" units="ms">
+  <owner>pmonette@chromium.org</owner>
+  <summary>
+    The time elapsed from the moment a tab was deactivated until it was
+    reactivated. Only recorded for tabs that are pinned or had form interaction.
+  </summary>
+</histogram>
+
+<histogram name="Tab.TimeToReactivation.Normal" units="ms">
+  <owner>pmonette@chromium.org</owner>
+  <summary>
+    The time elapsed from the moment a tab was deactivated until it was
+    reactivated. Only recorded for tabs that are not pinned nor had form
+    interaction.
+  </summary>
+</histogram>
+
 <histogram name="Tab.TotalTabCount.BeforeLeavingApp" units="tabs">
   <owner>jaekyun@chromium.org</owner>
   <summary>