Mojo: Make GetDefault{AsyncWaiter,Logger} static methods of Environment.

mojo/mojo_public.gypi

 {
   'targets': [
     {
       'target_name': 'mojo_system',
       'type': 'static_library',
       'defines': [
         'MOJO_SYSTEM_IMPLEMENTATION',
       ],
       'include_dirs': [
         '..',
@@ skipping 302 matching lines @@
       'dependencies': [
         'mojo_cpp_bindings',
       ],
     },
     {
       'target_name': 'mojo_environment_standalone',
       'type': 'static_library',
       'sources': [
         'public/c/environment/async_waiter.h',
         'public/c/environment/logger.h',
-        'public/cpp/environment/default_async_waiter.h',
-        'public/cpp/environment/default_logger.h',
         'public/cpp/environment/environment.h',
         'public/cpp/environment/lib/default_async_waiter.cc',
+        'public/cpp/environment/lib/default_async_waiter.h',
         'public/cpp/environment/lib/default_logger.cc',
+        'public/cpp/environment/lib/default_logger.h',
         'public/cpp/environment/lib/environment.cc',
       ],
       'include_dirs': [
         '..',
       ],
     },
     {
       'target_name': 'mojo_utility',
       'type': 'static_library',
       'sources': [
@@ skipping 92 matching lines @@
           },
           'dependencies': [
             'mojo_public_java',
           ],
           'includes': [ '../build/java.gypi' ],
         },
       ],
     }],
   ],
 }