Add back [ImplementedInBaseClass] IDL extended attribute

Originally [LegacyImplementedInBaseClass] was intended to be short-term,
as the name suggests, for some refactoring.

However, for more complex cases of 'implements' usage,
the requirements that 'implements' puts on C++ implementation
makes it an ugly mess.
Thus allowing implementation in base class seems useful long-term,
so re-introducing.

Per:
WebGLRenderingContextBase
https://codereview.chromium.org/205243013/

Documentation:
http://www.chromium.org/blink/webidl/blink-idl-extended-attributes#TOC-ImplementedInBaseClass-i-

Compare earlier CL removing this:
Remove support for [LegacyImplementedInBaseClass] IDL extended attribute 
https://codereview.chromium.org/169273002/

R=haraken
BUG=344224

Review URL: https://codereview.chromium.org/209663005

git-svn-id: svn://svn.chromium.org/blink/trunk@169830 bbb929c8-8fbe-4397-9dbb-9b2b20218538