Use ImageFrame:setRGBARaw for JPEG decoding

Remove an ASSERT from ImageFrame::setRGBARaw() to make it usable in
more circumstances. In particular, use it in the JPEG image decoder
for the very rare CMYK and RGB JPEG images cases to remove the code
branching caused by ImageFrame::setRGBA() in the row write loop.

Covered by existing tests, in particular:
  RGB jpeg: fast/images/rgb-jpeg-with-adobe-marker-only.html
  CMYK jpeg: fast/images/cmyk-jpeg-with-color-profile.html

BUG=

Review URL: https://codereview.chromium.org/17815002

git-svn-id: svn://svn.chromium.org/blink/trunk@153320 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert "Call PNGDecoder setHasAlpha only when needed"

Reverts commit f986b8e50aee94e25799e315295fdcdf5dc5d36f to prepare
the way for for the revert of blink r153004 (which regressed image
decode performance).

TBR=tonyg@chromium.org
BUG=254462

git-svn-id: svn://svn.chromium.org/blink/trunk@153321 bbb929c8-8fbe-4397-9dbb-9b2b20218538