Simplify/cleanup {postApply,apply}Resource signatures

With the scope of this interface reduced, one argument to
postApplyResource can be removed, and the GraphicsContext*& can be
simplified to GraphicsContext*. Also change "unsigned short resourceMode"
to RenderSVGResourceModeFlags while we're changing all signatures.

BUG=420022

Review URL: https://codereview.chromium.org/629073003

git-svn-id: svn://svn.chromium.org/blink/trunk@183273 bbb929c8-8fbe-4397-9dbb-9b2b20218538