Added Ash.TimeBetweenTaskSwitches to record time deltas between all recorded task switches.

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 785f32e3b791979c5f966b24b8584a261217ebaf..52b5dcca938f9c3541b1cddb92407efaa03da8e3 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -843,6 +843,20 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Ash.TimeBetweenTaskSwitches" units="seconds">
+  <owner>bruthig@google.com</owner>
+  <owner>tdanderson@google.com</owner>
+  <summary>
+    The number of seconds between contiguous task switch user actions triggered
+    by any of the other task switch actions that are tracked. (e.g.
+    Ash.Shelf.TimeBetweenNavigateToTaskSwitches,
+    Ash.Tab.TimeBetweenSwitchToExistingTabUserActions,
+    Ash.WindowCycleController.TimeBetweenTaskSwitches,
+    Ash.AppList.TimeBetweenTaskSwitches,
+    Ash.WindowSelector.TimeBetweenActiveWindowChanges, etc).

[tdanderson, 2015/06/02 15:56:12]

Since this list is (currently) exhaustive, perhaps change the e.g. to i.e. and
drop the etc. ?

[bruthig, 2015/06/04 17:53:30]

The reason I chose to have the 'etc' is because I doubt this description will be
kept up to date if a new task switch action is tracked.  I'd feel better keeping
it as an "e.g." and making the list non-exhaustive...  what do you think?

+  </summary>
+</histogram>
+
 <histogram name="Ash.TouchDuration" units="milliseconds">
   <obsolete>
     Deprecated 12/2013 in r239809, and replaced by Ash.TouchDuration2.