Add new gradient to support HTML Canvas2D two-point-radial-gradient. Ours is

Add new gradient to support HTML Canvas2D two-point-radial-gradient. Ours is
called (for now at least) TwoPointConical, to distinguish it from the
existing one (TwoPointRadial), and to reflect its odd behavior where it draws
transparent outside the boundary of the "cone" between the two circles.

This impl is unsupported by GPU and Printing at the moment, and Chrome does not
yet invoke it.
Review URL: https://codereview.appspot.com/6299051

git-svn-id: http://skia.googlecode.com/svn/trunk@4197 2bbb7eff-a529-9590-31e7-b0007b416f81