Make parseCompositeAndBlendOperator() take care of "normal".

Currently, parseCompositeAndBlendOperator() doesn't recognize "normal".
Fortunately, there are not bugs because all clients set blend mode to "normal"
if this function fails.

BUG=425656

Review URL: https://codereview.chromium.org/671713006

git-svn-id: svn://svn.chromium.org/blink/trunk@184239 bbb929c8-8fbe-4397-9dbb-9b2b20218538