[bindings] partial interfaces should not violate componentization

Design doc:
https://docs.google.com/a/google.com/document/d/1TpqG_bWJfHFo6swv5zbqfjxHl_K31C307NvdyF1NAcU/edit

https://docs.google.com/a/google.com/document/d/1TLOCAG1DrW7UdI_DVIclHeNn8rdHzDzK52gPPY8Uo5I/edit
(idea A: 
 core_idl_files: no partial interfaces in modules,
 core_idl_with_modules_dependency_files: partial interfaces in modules,
 webcore_testing_idl_files,
 webcore_testing_idl_with_modules_dependency_files: ditto)

BUG=380523

Review URL: https://codereview.chromium.org/618373003

git-svn-id: svn://svn.chromium.org/blink/trunk@184083 bbb929c8-8fbe-4397-9dbb-9b2b20218538