Revert r174105 - "Animated WebP: Alpha-blending support for semi-transparent pixels"

This failed to pass webkit_unit_tests on Android :(.

>
> Author: urvang@google.com <urvang@google.com@bbb929c8-8fbe-4397-9dbb-9b2b20218538>
> Date:   Thu May 15 20:47:22 2014 +0000

>    Animated WebP: Alpha-blending support for semi-transparent pixels
>
>    When alpha pre-multiplication is on, use Skia for blending.
>    Most of Blink (including image-rendering in the browser)
>    uses this mode.
>
>    When alpha pre-multiplication is off, use a locally written method.
>    Only WebGL image conversion uses this mode:
>    https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/platform/gr
>
>    Also, disable color profile handling for animated images. This is
>    an extremely rare use case and at the same time rather involved to
>    implement. Note: Color profiles in GIF images are ignored as well.
>
>    Testing:
>    Added a unit test to verify that decoded frames in case of alpha
>    pre-multiply on/off are equivalent.
>
>    Reviewers:
>    Mike Reed & Noel: For reviewing the alpha blending code.
>    Pascal: As a reviewer from WebP side.
>    Nico: For OWNER'S LGTM stamp.
>
>    BUG=364679
>
>    Review URL: https://codereview.chromium.org/274793003
>
>    git-svn-id: svn://svn.chromium.org/blink/trunk@174105 bbb929c8-8fbe-4397-9dbb-9b2b20218538

TBR=urvang@google.com
BUG=364679

Review URL: https://codereview.chromium.org/282323005

git-svn-id: svn://svn.chromium.org/blink/trunk@174111 bbb929c8-8fbe-4397-9dbb-9b2b20218538