Factor bindings.gypi

Prep for splitting bindings GYP files.
Currently bindings.gypi has a single long, qualified list of files.
It's simpler to put these in separate lists per directory,
which also lets the filenames be just the basename (unqualified).
This will simplify splitting into core vs. modules, and also makes
file organization more modular.

Tweaks to core.gyp:
the excess directories in 'include_dirs' are part of what I'll fix.

R=haraken
BUG=358074

Review URL: https://codereview.chromium.org/295403002

git-svn-id: svn://svn.chromium.org/blink/trunk@174780 bbb929c8-8fbe-4397-9dbb-9b2b20218538