Add metrics for how often APNGs are used

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 540d233bcb7d01c985ffcb88cdeccdff6f0ca59e..4f906140580e15abcb9b643f505e888d309f6a93 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -4764,6 +4764,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Blink.DecodedImage.APNG" enum="BooleanAPNG">
+  <owner>scroggo@chromium.org</owner>
+  <summary>
+    Whether a PNG image is actually an APNG. An APNG is a valid PNG image with
+    extra chunks for animation (https://wiki.mozilla.org/APNG_Specification).
+    This is logged once per image header decode, which typically happens twice
+    per PNG on the page.
+  </summary>
+</histogram>
+
 <histogram name="Blink.DecodedImage.CanvasExpanded"
     enum="BooleanCanvasExpanded">
   <obsolete>
@@ -82456,6 +82466,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Active"/>
 </enum>
 
+<enum name="BooleanAPNG" type="int">
+  <int value="0" label="Image is a normal PNG image"/>
+  <int value="1"
+      label="Image appears to be a valid APNG (valid acTL before IDAT)"/>
+</enum>
+
 <enum name="BooleanAttempted" type="int">
   <int value="0" label="Not Attempted"/>
   <int value="1" label="Attempted"/>