Replace GraphicsContext::setupPaintFor* with const getters.

This is simpler, and marginally more efficient (as it avoids setting up
the paint in a default state only to immediately overwrite it).

GraphicsContext::setupShader is declared but never defined, so its declaration has been removed while here.

BUG=381750

Review URL: https://codereview.chromium.org/311193009

git-svn-id: svn://svn.chromium.org/blink/trunk@175697 bbb929c8-8fbe-4397-9dbb-9b2b20218538