Add metrics for the number of attached displays.

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 01e6809e03ca3e89f81aeb43d388576ee5f0ddf2..0ff54deaeb1be50136c9afad35d40a52f319966b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -21304,6 +21304,22 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>The cat's flavor and how many bits there are in it.</summary>
 </histogram>
 
+<histogram name="OSX.DisplayCount.Changed">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    The number of displays present right after the user has attached or detached
+    a new display. Excludes mirrored displays, but includes sleeping displays.
+  </summary>
+</histogram>
+
+<histogram name="OSX.DisplayCount.StartUp">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    The number of displays present at start up. Excludes mirrored displays, but
+    includes sleeping displays.
+  </summary>
+</histogram>
+
 <histogram name="OSX.ExceptionHandlerEvents" enum="OSXExceptionHandlerEvents">
   <owner>mark@chromium.org</owner>
   <summary>Events seen by the OSX NSException swizzle.</summary>