Use GraphicContext::drawImageBuffer in FEBlend

Get rid of the "local" implementation of the blend modes in favor of the
Skia implementation.
Still uses the ARM NEON specific implementation for when
ARM_NEON_INTRINSICS is enabled.
Also change so that createImageFilter uses WebCoreCompositeToSkiaComposite
to map to the Skia Xfermode.

Inspired by WebKit commit http://trac.webkit.org/changeset/170433 by
Dirk Schulze.

BUG=389594

Review URL: https://codereview.chromium.org/416093002

git-svn-id: svn://svn.chromium.org/blink/trunk@178936 bbb929c8-8fbe-4397-9dbb-9b2b20218538