Match CanvasGradient::addColorStop to the spec.

The canvas spec [1] defines addColorStop without optional arguments. This patch
removes optional attribute from addColorStop interface.

[1] http://www.w3.org/TR/2dcontext2/

TEST=LayoutTests/canvas/philip/tests/2d.missingargs.html

Review URL: https://chromiumcodereview.appspot.com/14798006

git-svn-id: svn://svn.chromium.org/blink/trunk@152779 bbb929c8-8fbe-4397-9dbb-9b2b20218538
2 files changed