[JPEGDecoder] Use const version of isSizeAvailable()

Remove a const_cast on isSizeAvailable in canDecodeToYUV: call into the
base class instead to use its const isSizeAvailable method.

BUG=409375

Review URL: https://codereview.chromium.org/539513002

git-svn-id: svn://svn.chromium.org/blink/trunk@181360 bbb929c8-8fbe-4397-9dbb-9b2b20218538