Adds UMA histograms for desktop capture device.

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 4a482cf89b505cf6754ebb909ee735c14b1d73cf..024dae50c4c2fef4c610fc523cf1934cbb77bea9 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -30113,6 +30113,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.DesktopCaptureCounters" enum="DesktopCaptureCounters">
+  <owner>jiayl@chromium.org</owner>
+  <summary>
+    Counters on creation of DesktopCaptureDevice and the first capture call.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.NumDataChannelsPerPeerConnection">
   <owner>perkj@chromium.org</owner>
   <summary>
@@ -30151,6 +30158,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.ScreenCaptureTime">

[Ilya Sherman, 2014/04/24 01:57:43]

nit: Please add units.

+  <owner>jiayl@chromium.org</owner>
+  <summary>Time for capturing one frame in screen capturing.</summary>
+</histogram>
+
 <histogram name="WebRTC.SentAudioTrackDuration" units="milliseconds">
   <owner>perkj@chromium.org</owner>
   <summary>
@@ -30203,6 +30215,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.WindowCaptureTime">

[Ilya Sherman, 2014/04/24 01:57:43]

nit: Please add units.

+  <owner>jiayl@chromium.org</owner>
+  <summary>Time for capturing one frame in window capturing.</summary>
+</histogram>
+
 <histogram name="Webstore.ExtensionInstallResult" enum="BooleanSuccess">
   <owner>jackhou@chromium.org</owner>
   <summary>
@@ -31588,6 +31605,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="2" label="Proxy available and enabled"/>
 </enum>
 
+<enum name="DesktopCaptureCounters" type="int">
+  <int value="0" label="Screen capturer created."/>
+  <int value="1" label="Window capturer created."/>
+  <int value="2" label="First screen capture call succeeded."/>
+  <int value="3" label="First screen capture call failed."/>
+  <int value="4" label="First window capture call succeeded."/>
+  <int value="5" label="First window capture call failed."/>
+</enum>
+
 <enum name="DevicePermissionActions" type="int">
   <int value="0" label="AllowHttps"/>
   <int value="1" label="AllowHttp"/>