Add animation support for WebP images

LayoutTests/fast/images/webp-animated-large.html

Index: LayoutTests/fast/images/webp-animated-large.html
diff --git a/LayoutTests/fast/images/webp-animated-large.html b/LayoutTests/fast/images/webp-animated-large.html
new file mode 100644
index 0000000000000000000000000000000000000000..388c1a0c11c6e838883ab83e3e99f7b83f16324c
--- /dev/null
+++ b/LayoutTests/fast/images/webp-animated-large.html
@@ -0,0 +1,6 @@
+<script>
+if (window.testRunner)
+    window.testRunner.dumpAsText();
+</script>
+This test passes if no errors occur while decoding this large animation.<br>

[Noel Gordon, 2013/04/29 18:41:57]

How does this test work?  If there were decoding errors, I don't see how it
detects those errors.

[urvang (Google), 2013/04/30 13:14:15]

I had added this test hoping that Layout tests may be designed to catch memory
errors (for example, clearFrameBuffer() would be called only for large
animations), but perhaps not.

Now that I've added WEBPImageDecoderTest.cpp, which should be much more
comprehensive, we could remove this if you suggest so. And I could add this
image to be tested there.

[Noel Gordon, 2013/05/01 17:55:28]

The unit test is fine, but we also want a layout test since they get tested with
the ASAN and TSAN bots that check for memory leaks and threading issues,
respectively.

Suggest you look at fast/images/webp-image-decoding.html and crib something
similar for animated webp images.  Find three images to test with, for example,
the ones you added to fast/image/resources =)

+<img src="resources/webp-animated-large.webp">