Add ImageDecoder::isSizeAvailable() const

Image decoders override isSizeAvailable, which is non-const and correct
for the use-case. If callers require a const version, a const_cast must
used to workaround compiler errors. Instead, provide a const version of
isSizeAvailable(), non-virtual for now.

BUG=409375

Review URL: https://codereview.chromium.org/523373003

git-svn-id: svn://svn.chromium.org/blink/trunk@181134 bbb929c8-8fbe-4397-9dbb-9b2b20218538