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
Also:
- SkWindow now has createSurface, not createCanvas.
- Add the platform init code v8 now seems to require.
- Fix library linkage.
- Call isolate->Enter(); because it doesn't look
like v8 starts with a default isolate to begin with.
BUG=skia:
Review URL: https://codereview.chromium.org/673223002
