IDL: overload resolution for nullable types

4 interfaces:
AudioNode
HTMLSelectElement
URL
WebGLRenderingContext

Also fixes some IDL bugs, where there are invalid overloads with multiple nullable types (not distinguishable: what does null resolve to)?
Fixed these; no changes in behavior, since these were already resolved correctly to the first match.

R=haraken
BUG=293561

Review URL: https://codereview.chromium.org/274603004

git-svn-id: svn://svn.chromium.org/blink/trunk@174072 bbb929c8-8fbe-4397-9dbb-9b2b20218538