Use CanvasImageSource union type instead of overloading

In CanvasRenderingContext2D.idl, use the already defined union type
CanvasImageSource for the argument to drawImage() and createPattern(),
instead of having one overload of each per union member type.

This reduces the number of overloads for drawImage() from 12 to 3 and
thus greatly reduces the amount of generated bindings code for it.

BUG=430337

Review URL: https://codereview.chromium.org/703783004

git-svn-id: svn://svn.chromium.org/blink/trunk@185247 bbb929c8-8fbe-4397-9dbb-9b2b20218538