IDL: Support [ImplementedAs] in dictionaries

We need Blink-specific [ImplementedAs] extended attribute support
in dictionaries because a dictionary member can conflict with
C++ keywords or methods which are generated by the IDL compiler.
For example, FileSystem API defines a dictionary which contains
'create' member. This conflicts with a static factory method
'create()', which is generated by the IDL compiler.

BUG=321462

Review URL: https://codereview.chromium.org/621633002

git-svn-id: svn://svn.chromium.org/blink/trunk@183027 bbb929c8-8fbe-4397-9dbb-9b2b20218538