Added metrics to track the time between task switches done via the shelf.

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 4919d6689410bce54d43486f5d317bb14b4f1b58..f0e3d3c98aa18cf68f94a01701254e47b8cb71ed 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -729,6 +729,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Ash.Shelf.TimeBetweenNavigateToTaskSwitches" units="seconds">
+  <owner>bruthig@google.com</owner>
+  <owner>tdanderson@google.com</owner>
+  <summary>
+    The number of seconds between contiguous task switch user actions triggered

[tdanderson, 2015/05/13 19:27:18]

Perhaps clarify somehow that "contiguous" still means that other task switches
could take place in between, i.e., tapping the shelf, then tapping the tabstrip,
then tapping the shelf again will record an entry here.

[bruthig, 2015/05/13 21:16:21]

Updated.  I tried my best to be clear...

+    by the Shelf buttons where the user activates a different user-predicatble
+    task.
+  </summary>
+</histogram>
+
 <histogram name="Ash.ShelfAlignmentOverTime" enum="ShelfAlignmentValue">
   <owner>kuscher@google.com</owner>
   <summary>