IDL compiler: [PerContextEnabled] for getters

There is only one use of PerContextEnabled for attributes:
Source/core/page/WindowPagePopup.idl
[PerContextEnabled=PagePopup] readonly attribute PagePopupController pagePopupController;
...and 2 uses for operations:
Source/core/frame/History.idl

Longer-term, consider removing them.
It would be nice to get rid of these, as it's significant complexity to
support very limited use cases, but that would involve C++ changes.

I'm not implementing dual [RuntimeEnabled, PerContextEnabled]
or [Conditional, PerContextEnabled]
as these are not used in actual code;
also removing from Perl.

Other changes:
* Refactors attribute configuration in Jinja template into a macro,
since used 3 places
* Capitalizes Feature Names in PerContextEnabled values
* Fixes spacing in Perl CG

BUG=239771

Review URL: https://codereview.chromium.org/26547005

git-svn-id: svn://svn.chromium.org/blink/trunk@159774 bbb929c8-8fbe-4397-9dbb-9b2b20218538