IDL compiler: Extended attributes on partial interfaces

This adds support and tests for the 4 extended attributes on partial interfaces:
* [Conditional]
* [ImplementedAs]
* [PerContextEnabled]
* [RuntimeEnabled]
(Extra test file needed to avoid conflicts.)

This completes partial interfaces!

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

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/133273009

git-svn-id: svn://svn.chromium.org/blink/trunk@165296 bbb929c8-8fbe-4397-9dbb-9b2b20218538