Add a histogram for Blink decoded image type

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 f866bfbe3f88ba4da8303ea4274ed17bf365ff5e..316c9bd3f82a20bdbff7e8140561f4ce3fb1ffd0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -2601,6 +2601,10 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Blink.DecodedImageType" enum="DecodedImageType">
+  <summary>Image codec inferred during decode.</summary>

[Alexei Svitkine (slow), 2015/07/29 15:54:08]

Nit: Mention that this ignores trivial 1x1 images, as you do in your
corresponding CL.

[urvang, 2015/07/29 16:03:22]

Done.

+</histogram>
+
 <histogram name="Blink.MediaElement.Autoplay" enum="MediaElementAutoPlay">
   <owner>oysteine@chromium.org</owner>
   <summary>
@@ -53159,6 +53163,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
       label="Arrived at settings menu by another path: entered on, exited on"/>
 </enum>
 
+<enum name="DecodedImageType" type="int">
+  <int value="0" label="kImageUnknown"/>
+  <int value="1" label="kImageJPEG"/>
+  <int value="2" label="kImagePNG"/>
+  <int value="3" label="kImageGIF"/>
+  <int value="4" label="kImageWebP"/>
+  <int value="5" label="kImageICO"/>
+  <int value="6" label="kImageBMP"/>
+</enum>
+
 <enum name="DefaultBrowserState" type="int">
   <int value="0" label="Not Default"/>
   <int value="1" label="Default"/>