Add ImageDecoder::deferredImageDecodingEnabled() method

Allow platform ImageDecoder to determine if deferred image decoding
is enabled without needing to depend on Page settings. No change in
behavior, so no new tests.

TBR=kbr@chromium.org
BUG=360386

Review URL: https://codereview.chromium.org/227113004

git-svn-id: svn://svn.chromium.org/blink/trunk@171200 bbb929c8-8fbe-4397-9dbb-9b2b20218538