Add histograms to record the time since tab was last active.

histograms/histograms.xml

Index: histograms/histograms.xml
===================================================================
--- histograms/histograms.xml	(revision 278704)
+++ histograms/histograms.xml	(working copy)
@@ -10871,6 +10871,22 @@
   </summary>
 </histogram>
 
+<histogram name="MemoryWarning.EvictedTabTimeSinceActive" units="ms">
+  <owner>lliabraa@chromium.org</owner>
+  <summary>
+    [iOS] When the OS sends a memory warning and the app evicts a tab, this
+    histogram records the time since the evicted tab was active.
+  </summary>
+</histogram>
+
+<histogram name="MemoryWarning.ProtectedTabTimeSinceActive" units="ms">
+  <owner>lliabraa@chromium.org</owner>
+  <summary>
+    [iOS] When the OS sends a memory warning and the app protects a tab, this
+    histogram records the time since the protected tab was active.
+  </summary>
+</histogram>
+
 <histogram name="Mist.SwitchResult" enum="MistSwitchResult">
   <owner>benchan@chromium.org</owner>
   <summary>
@@ -30254,6 +30270,16 @@
   </summary>
 </histogram>
 
+<histogram name="Tab.EvictedTabWasActive" enum="Boolean">
+  <owner>lliabraa@chromium.org</owner>
+  <summary>
+    [iOS] When switching to an evicted tab, this histogram records whether or
+    not the tab had ever been active. For example, the tab was opened via
+    &quot;Open in new tab&quot; but evicted before being viewed for the first
+    time.
+  </summary>
+</histogram>
+
 <histogram name="Tab.FormActivityCountEvictedHistogram">
   <owner>lliabraa@chromium.org</owner>
   <summary>
@@ -30372,6 +30398,22 @@
   </summary>
 </histogram>
 
+<histogram name="Tab.TimeSinceActive" units="ms">
+  <owner>lliabraa@chromium.org</owner>
+  <summary>
+    [iOS] When an existing tab becomes active, this histogram records the time
+    since it was made inactive.
+  </summary>
+</histogram>
+
+<histogram name="Tab.TimeSinceActiveEvicted" units="ms">
+  <owner>lliabraa@chromium.org</owner>
+  <summary>
+    [iOS] When an evicted tab becomes active, this histogram records the time
+    since it was made inactive.
+  </summary>
+</histogram>
+
 <histogram name="Tab.TimeSinceFormActivityEvictedHistogram" units="ms">
   <owner>lliabraa@chromium.org</owner>
   <summary>