Pass SharedBuffer by reference to DeferredImageDecoder::setData()

Pass SharedBuffer by reference to DeferredImageDecoder::setData() as it can
never be null. This gets rid of a few unnecessary null checks. The existing
code would have crashed anyway if the pointer were null as 1 null check was
missing.

R=abarth@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/244243002

git-svn-id: svn://svn.chromium.org/blink/trunk@172000 bbb929c8-8fbe-4397-9dbb-9b2b20218538