IDL build: Split global objects computation from global constructors computation

Preliminary CL for splitting global constructor computation between core
and modules.
Straightforward, but needed for pipeline.

Computation of global constructors (as partial interface definitions) depends
on global *objects* (interface definitions with [Global] or [PartialGlobal]).

We want to split global *constructors* between core and modules (as separate
partial interface .idl files), but we first need to compute global *objects*.
Thus we split this computation into 2 steps.

Also a bit of cleanup.

Followup CLs:
* Split global constructors computation between core and modules
* Split global objects computation between core and modules
...at which point the bindings build will be 100% split!

R=haraken
BUG=358074

Review URL: https://codereview.chromium.org/309793002

git-svn-id: svn://svn.chromium.org/blink/trunk@175267 bbb929c8-8fbe-4397-9dbb-9b2b20218538