Start moving to the new canvas structure.

Adds DrawingMethods with some of the methods it defines. Context is now
an implementation of DrawingMethods.

The sample.js file now shows how the context is used.

Not much new code here, that's mostly in DrawingMethods::DrawPath, most everything else is a code move.

BUG=skia:

Review URL: https://codereview.chromium.org/676423002
7 files changed