Move existing Mojo JS <--> CPP tests out of webui test harness.

mojo/mojo_public.gypi

 {
   'targets': [
     {
       'target_name': 'mojo_system',
       'type': 'static_library',
       'defines': [
         'MOJO_SYSTEM_IMPLEMENTATION',
       ],
       'include_dirs': [
         '..',
@@ skipping 252 matching lines @@
       ],
       'sources': [
         'public/js/bindings/constants.cc',
         'public/js/bindings/constants.h',
       ],
     },
     {
       'target_name': 'mojo_public_test_interfaces',
       'type': 'static_library',
       'sources': [
+        'public/interfaces/bindings/tests/js_to_cpp.mojom',
         'public/interfaces/bindings/tests/math_calculator.mojom',
         'public/interfaces/bindings/tests/sample_factory.mojom',
         'public/interfaces/bindings/tests/sample_import.mojom',
         'public/interfaces/bindings/tests/sample_import2.mojom',
         'public/interfaces/bindings/tests/sample_interfaces.mojom',
         'public/interfaces/bindings/tests/sample_service.mojom',
         'public/interfaces/bindings/tests/test_structs.mojom',
       ],
       'variables': {
         'mojom_base_output_dir': 'mojo',
@@ skipping 94 matching lines @@
           'type': 'none',
           'variables': {
             'java_in_dir': 'public/java',
           },
           'includes': [ '../build/java.gypi' ],
         },
       ],
     }],
   ],
 }