Fix miter limit default for 2d canvas.

Per http://www.w3.org/html/wg/drafts/2dcontext/html5_canvas/, miter
limit should default to 10. CanvasRenderingContext2D::State uses this
default in its constructor; however, GraphicsContext's StrokeData,
which also stores miter limit, defaults to 4. This mismatch caused
several issues:
1) The default miter limit that Blink used for drawing in a 2d canvas
   was incorrect.
2) A 2d canvas context would report an incorrect miter limit.
3) Trying to force the miter limit to 10 by setting it would fail
   unless it was first set to another value.

BUG=225512

Review URL: https://chromiumcodereview.appspot.com/18420010

git-svn-id: svn://svn.chromium.org/blink/trunk@154143 bbb929c8-8fbe-4397-9dbb-9b2b20218538