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

[Noel Gordon, 2013/05/01 19:11:41]

As suggested, crib your layout test from fast/images/webp-image-decoding.html to
appease our ASAN/TSAN bots.

[urvang (Google), 2013/05/01 22:25:16]

Oh these run under ASAN/TSAN, good.

However, just realized that these tests won't work as 
"RuntimeEnabledFeatures::animatedWebPEnabled()" is false by default. (Not sure
how it was working so far).

Question: Is there a way to enable this flag for layout tests (similar to the
way it is done in unit tests)?

Otherwise, I'm just putting test expectation to be 'failure' for now.

[Noel Gordon, 2013/05/02 18:00:27]

http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20ASAN for
example.
Yes, and the enable will soon be auto-generated according to Eric.  Failing
that, you modify InternalSettings to allow a layout test to set a runtime flag. 
For example,

  % grep CSSVariables Source/core/testing/InternalSettings.*

and see LayoutTests/fast/css/variables/invalid-font-reference.html
Seems fine for now. Expect news from Eric.