Merge idl_definitions_builder.py into idl_definitions.py

idl_definitions_builder.py was separate from idl_definitions.py
b/c the latter had lots of Perl compatibility cruft (JSON export etc.)
Now that it's gone, there's no reason to have these separate, as
idl_definitions was almost only class definitions, and
idl_definitions_builder was basically constructors.

Keeping them separate made it harder to read (b/c had to bounce back
and forth), and the calls to the actual constructors were incredibly
long.

This merges them, deleting one file and making the code quite simple.
Net -70 lines, though more like -100 due to adding a few comments
(class diagram).

Change is just merging the functions into constructors (in a few cases
changing an alternative constructor into a class method) and adding
a few comments.

Further cleanup is possible (remove IdlEnum and IdlUnionType),
but involve CG changes, so will do in followup.

BUG=345137
TBR=haraken

Review URL: https://codereview.chromium.org/177233006

git-svn-id: svn://svn.chromium.org/blink/trunk@168582 bbb929c8-8fbe-4397-9dbb-9b2b20218538