IDL: overload resolution for optional distinguishing argument

Simple change, and lets us eliminate [LegacyOverloadString]!

2 interfaces:
CanvasRenderingContext2D
Navigator

* Updated test case
* Updated docs
http://www.chromium.org/blink/webidl/blink-idl-extended-attributes#TOC-Temporary-Blink-specific-IDL-Extended-Attributes

R=haraken
BUG=293561

Review URL: https://codereview.chromium.org/273813008

git-svn-id: svn://svn.chromium.org/blink/trunk@174065 bbb929c8-8fbe-4397-9dbb-9b2b20218538