Mojo: Split off the C++ parts of mojo/public/c/system macros.h to their own file.

mojo/mojo_public.gypi

 {
   'targets': [
     {
       'target_name': 'mojo_system',
       'type': 'shared_library',
       'defines': [
         'MOJO_SYSTEM_IMPLEMENTATION',
       ],
       'include_dirs': [
         '..',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '..',
         ],
       },
       'sources': [
         'public/c/system/async_waiter.h',
         'public/c/system/core.h',
         'public/c/system/macros.h',
         'public/c/system/system_export.h',
         'public/cpp/system/core.h',
+        'public/cpp/system/macros.h',
         'public/system/core_private.cc',
         'public/system/core_private.h',
       ],
       'conditions': [
         ['OS=="mac"', {
           'xcode_settings': {
             # Make it a run-path dependent library.
             'DYLIB_INSTALL_NAME_BASE': '@rpath',
           },
           'direct_dependent_settings': {
@@ skipping 153 matching lines @@
         '../base/base.gyp:base',
         '../testing/gtest.gyp:gtest',
         'mojo_bindings',
         'mojo_public_test_utils',
         'mojo_run_all_unittests',
         'mojo_system',
       ],
       'sources': [
         'public/c/tests/system/core_unittest.cc',
         'public/c/tests/system/core_unittest_pure_c.c',
+        'public/c/tests/system/macros_unittest.cc',
         'public/cpp/tests/system/core_unittest.cc',
-        'public/tests/system/macros_unittest.cc',
+        'public/cpp/tests/system/macros_unittest.cc',
       ],
     },
     {
       'target_name': 'mojo_public_utility_unittests',
       'type': 'executable',
       'dependencies': [
         '../base/base.gyp:base',
         '../testing/gtest.gyp:gtest',
         'mojo_bindings',
         'mojo_public_test_utils',
@@ skipping 175 matching lines @@
       ],
       'dependencies': [
         'mojo_shell_bindings',
       ],
       'export_dependent_settings': [
         'mojo_shell_bindings',
       ],
     },
   ],
 }