Add UMA histograms for data usage tab model

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 b76f7b8d8e9058c0513800cd55351827c2ae75ac..52271e483fc0ab8405c0b97a583c3170db4c8660 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6423,6 +6423,86 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="DataUse.TabModel.ExpiredActiveTabEntryRemovalDuration"
+    units="seconds">

[tbansal1, 2015/11/18 06:59:57]

UMA_HISTOGRAM_CUSTOM_TIMES is always in milliseconds. 
https://code.google.com/p/chromium/codesearch#chromium/src/base/metrics/histo...

So, s/seconds/milliseconds/
here and in other histograms below.

[Raj, 2015/11/18 16:56:28]

Done.

+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The duration in seconds from the start time of a tab entry's latest data

[tbansal1, 2015/11/18 06:59:58]

Remove "in seconds" from the description since the units are already specified
separately.
Here, and in other descriptions below.

[Raj, 2015/11/18 16:56:28]

Done.

+    usage tracking session, till when the entry is expired and removed from the
+    tab model. This is for tab entries that are still actively tracking data
+    usage for a Chrome tab.

[tbansal1, 2015/11/18 06:59:58]

s/Chrome/Chromium
Here, and below

[Raj, 2015/11/18 16:56:28]

Done.

+
+    Tab model maintains the tab entries, each pertaining to tracking sessions of
+    a single Chrome tab. These entries are considered expired after some
+    duration and can be removed from the tab model after that. The sample is
+    taken when an expired tab entry still tracking data usage is removed from
+    the tab model.
+  </summary>
+</histogram>
+
+<histogram name="DataUse.TabModel.ExpiredInactiveTabEntryRemovalDuration"
+    units="seconds">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The duration in seconds from the end time of a tab entry's latest data usage
+    tracking session, till when the entry is expired and removed from the tab
+    model. This is for tab entries that are not actively tracking data usage for
+    a Chrome tab.
+
+    Tab model maintains the tab entries, each pertaining to tracking sessions of
+    a single Chrome tab. These entries are considered expired after some
+    duration and can be removed from the tab model after that. The sample is
+    taken when an expired inactive tab entry is removed from the tab model.
+  </summary>
+</histogram>
+
+<histogram name="DataUse.TabModel.OldInactiveSessionRemovalDuration"
+    units="seconds">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The duration in seconds from the end time of a data usage tracking session,
+    till when the tracking session is removed from the tab model. This is for
+    old tracking sessions that are not actively tracking data usage.
+
+    Tab model maintains a limited number of tracking sessions per Chrome tab.
+    When this limit is reached oldest inactive tracking sessions are removed.
+    The sample is taken when an old inactive tracking session of a tab is
+    removed from the tab model.
+  </summary>
+</histogram>
+
+<histogram name="DataUse.TabModel.TrackingSessionLifetime" units="seconds">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The lifetime of a data usage tracking session of a Chrome tab in seconds,
+    maintained by the tab model. This is the duration between the time tracking
+    started due to an entry event and ended with an exit event. The sample is
+    taken when the data usage tracking session ends.
+  </summary>
+</histogram>
+
+<histogram name="DataUse.TabModel.UnexpiredTabEntryRemovalDuration"
+    units="seconds">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The duration in seconds from the end time (start time, if still active) of a
+    tab entry's latest data usage tracking session, till when the entry is
+    removed from the tab model. This is for tab entries that are not expired
+    yet, and removed from the tab model.
+
+    Tab model maintains a limited number of entries, each pertaining to tracking
+    sessions of a single Chrome tab. When this limit is reached even after
+    removing expired tab entries, then oldest unexpired tab entries are removed.
+    The sample is taken when an unexpired tab entry is removed from the tab
+    model.
+  </summary>
+</histogram>
+
 <histogram name="DataUse.TrafficSize.System" units="bytes">
   <owner>amohammadkhan@chromium.org</owner>
   <owner>bengr@chromium.org</owner>