IDL compiler: [ImplementedAs] for getters

Partly implemented already.

Docs:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-ImplementedAs-i-m-a-

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/26786002

git-svn-id: svn://svn.chromium.org/blink/trunk@159654 bbb929c8-8fbe-4397-9dbb-9b2b20218538