Named getter of HTMLAllCollection etc should be nullable

According to the spec[1,2,3], named getter of HTMLAllCollection, HTMLFormControlsCollection and HTMLOptionsCollection should be nullable.

This CL also adds layout tests for nullable arguments.

[1] https://html.spec.whatwg.org/multipage/infrastructure.html#the-htmlallcollection-interface
[2] https://html.spec.whatwg.org/multipage/infrastructure.html#the-htmlformcontrolscollection-interface
[3] https://html.spec.whatwg.org/multipage/infrastructure.html#htmlallcollection

BUG=430337

Review URL: https://codereview.chromium.org/692993003

git-svn-id: svn://svn.chromium.org/blink/trunk@185108 bbb929c8-8fbe-4397-9dbb-9b2b20218538