Add clearColorTransform() routine to JPEG/PNG/WEBP decoders

Resetting the color transform is required in two places in these
decoders. Define a helper function and call it to do reset. 
 
BUG=404639

Review URL: https://codereview.chromium.org/480063003

git-svn-id: svn://svn.chromium.org/blink/trunk@180521 bbb929c8-8fbe-4397-9dbb-9b2b20218538