Add SVG as a picture supported mime type

Add a method that checks a type string against the list of supported images as well as SVG.

BUG=398443

Review URL: https://codereview.chromium.org/430583002

git-svn-id: svn://svn.chromium.org/blink/trunk@180912 bbb929c8-8fbe-4397-9dbb-9b2b20218538