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

Adds histograms to record the time since a tab was active when:
 - switching to the tab
 - switching to an evicted tab
 - protecting a tab on memory warning
 - evicting a tab on memory warning

Also adds a histogram to show how often users switch to
an evicted tab that was never active.

BUG=385111
R=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/340113003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284061 0039d316-1c4b-4281-b951-d872f2087c98

tools/metrics/histograms/histograms.xml

@@ -11254,6 +11254,22 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </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>
@@ -32154,6 +32170,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </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>
@@ -32272,6 +32298,22 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </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>