Split dependencies from built IDL files in GYP

This statically separates:
* actual target IDL files (for which .h/.cpp files are generated)
from:
* dependency IDL files (for which dummy files are generated)
...which significantly reduces the work done in generating bindings and
allows us to remove lots of code and complexity (b/c these aren't distinguished).

Improvements:
* Reduces generated bindings by 9% (1348 => 1236)
* Reduces build edges by 9% (678 => 618)
* Reduces tests/results by 10% (61 => 55)
* Removes --additionalIdlFiles parameter and code
* Removes dummy file generation code

Further simplification is possible:
* compute_dependencies
  (currently has to remove dependencies, and treat main and support separately)
* derived_sources_all_in_one
  Possibly can be eliminated or significantly simplified, as we now have
  the full list (it's just main_interface_idl_files)
* idl_compiler.py & code_generator_v8.py, since guaranteed to have real IDL file
* Perhaps remove the 'support_*' variables (used by testing)

I'll do these in followup CLs.

Review URL: https://codereview.chromium.org/136033011

git-svn-id: svn://svn.chromium.org/blink/trunk@165289 bbb929c8-8fbe-4397-9dbb-9b2b20218538