Rename [TypeChecking=String] => [LegacyOverloadString]

[TypeChecking=String] (formerly [StrictTypeChecking] on a DOMString arg)
is not type checking: it's overload resolution.

Splitting this out keeps [TypeChecking] only for actual type checking.

Background:
This was needed in the past b/c Web IDL didn't support overloading
DOMString and float (these were not "distinguishable").
However, this has been changed in the spec, so this will be default
behavior when I rewrite overload resolution.

Docs updated:
http://www.chromium.org/blink/webidl/blink-idl-extended-attributes#TOC-TypeChecking-i-m-a-
http://www.chromium.org/blink/webidl/blink-idl-extended-attributes#TOC-LegacyOverloadString-p-

R=haraken
BUG=293561

Review URL: https://codereview.chromium.org/255763003

git-svn-id: svn://svn.chromium.org/blink/trunk@172592 bbb929c8-8fbe-4397-9dbb-9b2b20218538