Split generated_bindings.gyp into core and modules (1/2)

This splits the files, except for generating individual bindings
(that's long, so I'll do it separately).

This is a bit big b/c it's refactoring, but it's just splitting things up, and isolating core and modules from complexity in bindings.

Also adds FIXMEs with links to bugs for followups.

Immediate followups:
1. Split off individual binding generation (completes split at GYP level)
2. Split off/cleanup script GYP (currently a bit intricate, will simplify).

R=haraken
TBR=Nico
BUG=358074

Review URL: https://codereview.chromium.org/300133002

git-svn-id: svn://svn.chromium.org/blink/trunk@174833 bbb929c8-8fbe-4397-9dbb-9b2b20218538