Track positions of dimensions in image files

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 b5a898b6df1e8505268736b6a8fad01ac19d7137..54fbf64a32ff0d3582ae35bb87a9119619b58d94 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3091,6 +3091,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Blink.Images.SizeOffset">

[Alexei Svitkine (slow), 2016/02/01 20:51:11]

Please add units="" to these.

+  <owner>bengr@chromium.org</owner>
+  <summary>
+    The number of image file bytes that need to be retrieved in order to decode
+    the image dimensions, using the current, non-optimized implementation. The
+    suffix specifies the image type.
+  </summary>
+</histogram>
+
+<histogram name="Blink.Images.SizeOffsetOptimized">
+  <owner>bengr@chromium.org</owner>
+  <summary>
+    The minimum number of image file bytes that need to be retrieved in order to
+    decode the image dimensions. Reported only when this minimum is less the the
+    number of bytes needed using the current image reader implementation. The
+    suffix specifies the image type.
+  </summary>
+</histogram>
+
 <histogram name="Blink.MediaElement.Autoplay" enum="MediaElementAutoPlay">
   <owner>oysteine@chromium.org</owner>
   <summary>
@@ -83125,6 +83144,13 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="PLT.BeginToFinish_NormalLoad"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="ImageType">
+  <suffix name="GIF"/>
+  <suffix name="JPEG"/>
+  <affected-histogram name="Blink.Images.SizeOffset"/>
+  <affected-histogram name="Blink.Images.SizeOffsetOptimized"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="IMEAutoCorrect" separator=".">
   <suffix name="AC0" label="The auto-correct level is 0"/>
   <suffix name="AC1" label="The auto-correct level is 1"/>