Streamline the GraphicsContext annotation API.

As more annotation modes are being added, the current/typed API grows
unwieldy.

We should operate on an opaque list of string pairs at the GC level.

R=schenney@chromium.org,caseq@chromium.org

Review URL: https://codereview.chromium.org/373143002

git-svn-id: svn://svn.chromium.org/blink/trunk@177682 bbb929c8-8fbe-4397-9dbb-9b2b20218538