Record UMA stats for Blink decoded image types

Source/platform/graphics/BitmapImage.h

Index: Source/platform/graphics/BitmapImage.h
diff --git a/Source/platform/graphics/BitmapImage.h b/Source/platform/graphics/BitmapImage.h
index 6d49ff74f2bb1670bab47930ca08a0bb50066e70..59a47baa83ddce259dce36c57e988bd508eb5e9b 100644
--- a/Source/platform/graphics/BitmapImage.h
+++ b/Source/platform/graphics/BitmapImage.h
@@ -41,6 +41,23 @@
 
 namespace blink {
 
+enum DecodedImageType {
+    // These values are histogrammed over time; do not change their ordinal

[Noel Gordon, 2015/08/01 02:33:06]

We should move this into its own file, similar to
platform/image-decoder/ImageAnimation.h and
platform/graphics/ImageAnimimationPolicy.h

Please add FIXME: to do that in a follow-up patch.

[urvang, 2015/08/04 20:21:17]

Can you explain why this is necessary?
- Other enums in chrome://histograms/Blink don't seem to be in their own files.
See:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...
for example.
- For ImageAnimationPolicy, a separate file made sense as it's used at multiple
places. The same doesn't apply to this enum.

+    // values.  When deleting an image type, replace it with a dummy value;
+    // when adding an image type, do so at the bottom (and update DecodedImageTypeMax).
+    // Also, this enum should be in sync with the 'DecodedImageType' enum in
+    // tools/metrics/histograms/histograms.xml

[Noel Gordon, 2015/08/01 02:33:06]

tools/metrics/histograms/histograms.xml ->
src/tools/metrics/histograms/histograms.xml

[urvang, 2015/08/04 20:21:16]

In code search, "tools/metrics/histograms/histograms.xml" returns 52 results,
while "src/tools/metrics/histograms/histograms.xml" returns 6 results. So former
seems to be the norm.
But I'm OK either way.

Edited.

+    ImageUnknown = 0,
+    ImageJPEG,
+    ImagePNG,
+    ImageGIF,
+    ImageWebP,
+    ImageICO,
+    ImageBMP,
+
+    DecodedImageTypeMax = ImageBMP // Must equal the last "real" image type above.

[Noel Gordon, 2015/08/01 02:33:06]

nit: abbv DecodedImageTypeMax.  Rename it to LastDecodedImageType, drop the
comment here, amend the main comment where it refers to DecodedImageTypeMax.

[urvang, 2015/08/04 20:21:16]

Done.

+};
+
 template <typename T> class Timer;
 
 class PLATFORM_EXPORT BitmapImage final : public Image {