Add mojo::StrongBindingSet and use it in GeolocationServiceContext.

mojo/mojo_base.gyp

Index: mojo/mojo_base.gyp
diff --git a/mojo/mojo_base.gyp b/mojo/mojo_base.gyp
index 2eb3651acbd5000e0858f2f3cc15c3f4871f0bd1..e5b1fe9982569fd59432d39e2730133c93d4871c 100644
--- a/mojo/mojo_base.gyp
+++ b/mojo/mojo_base.gyp
@@ -81,6 +81,7 @@
         'common/data_pipe_file_utils.cc',
         'common/data_pipe_utils.cc',
         'common/data_pipe_utils.h',
+        'common/strong_binding_set.h',
       ],
     },
     {
@@ -134,6 +135,7 @@
         '../third_party/mojo/mojo_edk.gyp:mojo_common_test_support',
         '../third_party/mojo/mojo_edk.gyp:mojo_run_all_unittests',
         '../third_party/mojo/mojo_public.gyp:mojo_cpp_bindings',
+        '../third_party/mojo/mojo_public.gyp:mojo_public_test_interfaces_mojom',
         '../third_party/mojo/mojo_public.gyp:mojo_public_test_utils',
         'mojo_common_lib',
         'mojo_environment_chromium',
@@ -142,6 +144,7 @@
       ],
       'sources': [
         'common/common_type_converters_unittest.cc',
+        'common/strong_binding_set_unittest.cc',
         'message_pump/handle_watcher_unittest.cc',
         'message_pump/message_pump_mojo_unittest.cc',
       ],