Added UMA statistics for changing the active window via click or touch events.

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..fa0cbf18dd3b9b80c399ca31bcc1744b59328057 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -623,6 +623,20 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Ash.Desktop.TimeBetweenNavigateToTaskSwitches" units="seconds">
+  <owner>bruthig@chromium.org</owner>
+  <owner>tdanderson@chromium.org</owner>
+  <summary>
+    The number of seconds between task window activations triggered by users
+    clicking or tapping on a window in the 2-dimensional screen space. Task
+    switches from other sources are ignored and do not affect this metric. In
+    other words, if a user performs the following steps (1) click on task window
+    'A' (2) Alt+Tab to task 'Z'(3) click on task window 'B', then this will
+    result in a sample recorded for the time delta between activating task
+    window 'A' and 'B'.
+  </summary>
+</histogram>
+
 <histogram name="Ash.Dock.Action" enum="DockedAction">
   <owner>kuscher@google.com</owner>
   <owner>varkha@chromium.org</owner>