Record UMA stats for Blink decoded image orientation

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 83f6ec74d3370d1bb191b6e737d6446990429147..71a2a41f92cce45ffea1f529a74d7d90ce4bb670 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -2914,6 +2914,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Blink.DecodedImageOrientation" enum="DecodedImageOrientation">

[Alexei Svitkine (slow), 2015/11/17 22:00:25]

Nit: Suggest adding another . in the name:

Blink.DecodedImage.Orientation

This makes it slightly more readable and will group things together in case new
histograms about the decoded image are added in the future. (If you change it,
don't forgot to change both here and in the code.)

+  <owner>rob.buis@samsung.org</owner>
+  <summary>Image orientation inferred during decode.</summary>
+</histogram>
+
 <histogram name="Blink.MediaElement.Autoplay" enum="MediaElementAutoPlay">
   <owner>oysteine@chromium.org</owner>
   <summary>
@@ -56837,6 +56842,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="6" label="kImageBMP"/>
 </enum>
 
+<enum name="DecodedImageOrientation" type="int">
+  <int value="0" label="kImageOrientationUnknown"/>

[Alexei Svitkine (slow), 2015/11/17 22:00:25]

Nit: Make these names human readable, e.g. "Top Left", "Top Right", etc. Since
they'll be shown on the dashboard.

+  <int value="1" label="kImageOrientationTopLeft"/>
+  <int value="2" label="kImageOrientationTopRight"/>
+  <int value="3" label="kImageOrientationBottomRight"/>
+  <int value="4" label="kImageOrientationBottomLeft"/>
+  <int value="5" label="kImageOrientationLeftTop"/>
+  <int value="6" label="kImageOrientationRightTop"/>
+  <int value="7" label="kImageOrientationRightBottom"/>
+  <int value="8" label="kImageOrientationLeftBottom"/>
+</enum>
+
 <enum name="DefaultBrowserAsyncAttemptResult" type="int">
   <int value="0" label="Success">No errors encountered.</int>
   <int value="1" label="Already default">