Clarify DeferredImageDecoder isSizeAvailable() comment

Comment only change: grammar fix.

NOTREECHECKS=true
NOTRY=true
BUG=404544

Review URL: https://codereview.chromium.org/591093002

git-svn-id: svn://svn.chromium.org/blink/trunk@182418 bbb929c8-8fbe-4397-9dbb-9b2b20218538