Image decoders don't need to check for big-endian anymore

Chromium is designed for little-endian machines, and these have BGRA
pixel order (or RGBA on chrome Android), refer to:

https://bugs.webkit.org/show_bug.cgi?id=59670#c10
https://bugs.webkit.org/show_bug.cgi?id=72234
https://bugs.webkit.org/show_bug.cgi?id=88613

TEST=Covered by existing JPEG/WEBP layout tests.
BUG=None

Review URL: https://codereview.chromium.org/15989004

git-svn-id: svn://svn.chromium.org/blink/trunk@159760 bbb929c8-8fbe-4397-9dbb-9b2b20218538