Remove ImageSource::bytesDecodedToDetermineProperties() and fallout

Since the WebKit split, it always returns zero. Removing it enables us
also to remove the not always up-to-date m_decodedSize and avoid a
quite common assertion failure on GIF animations. No functional
changes expected.

BUG=288879

Review URL: https://codereview.chromium.org/25976002

git-svn-id: svn://svn.chromium.org/blink/trunk@159433 bbb929c8-8fbe-4397-9dbb-9b2b20218538