Remove convertToSkiaFilterLevel()

Of the 2 callers one doesn't really need the conversion and  the other can get
the same results with a local flag.

BUG=

Review URL: https://codereview.chromium.org/412353003

git-svn-id: svn://svn.chromium.org/blink/trunk@178987 bbb929c8-8fbe-4397-9dbb-9b2b20218538