Bindings build: split compute_interfaces_info into 2 stages

Preparatory refactoring: no changes to generated bindings.

Next step in splitting bindings build is splitting compute_interfaces_info
into core and modules.

For several reasons, I'm *first* splitting the compute_interfaces_info step
into two steps (vertical split): individual and overall.
In follow-up I'll split the individual step in two – core and modules
(horizontal split) – but to keep it simple I'll do that separately.

The reasons are:
1. To treat different IDL files differently, depending on component;
(In follow-up I'll split individual into core vs. modules, and then we'll
finally be able to distinguish components in the CG, and generate qualified
include paths w/o hideous path hacks!)

2. To avoid duplication (we want to avoid reading in all core IDL files for
core, and then reading in all core IDL files *again* and module IDL files for
modules);

3. Simplify build, by having simpler steps.

R=haraken
TBR=Nico
BUG=358074

Review URL: https://codereview.chromium.org/300273005

git-svn-id: svn://svn.chromium.org/blink/trunk@174935 bbb929c8-8fbe-4397-9dbb-9b2b20218538