Add mojo::StrongBindingSet and use it in GeolocationServiceContext.

mojo/common/BUILD.gn

Index: mojo/common/BUILD.gn
diff --git a/mojo/common/BUILD.gn b/mojo/common/BUILD.gn
index 0d8f3554c06c93427b30add4c32f5f882db1e83b..c0b935dfe97c062696617a3728561d91c9bc9188 100644
--- a/mojo/common/BUILD.gn
+++ b/mojo/common/BUILD.gn
@@ -23,6 +23,7 @@ component("common_base") {
     "data_pipe_file_utils.cc",
     "data_pipe_utils.cc",
     "data_pipe_utils.h",
+    "service_set.h",
     "user_agent.cc",
     "user_agent.h",
     "weak_binding_set.h",
@@ -84,6 +85,7 @@ test("mojo_common_unittests") {
     "//third_party/mojo/src/mojo/edk/test:test_support",
     "//third_party/mojo/src/mojo/public/cpp/bindings",
     "//third_party/mojo/src/mojo/public/cpp/test_support:test_utils",
+    "//third_party/mojo/src/mojo/public/interfaces/bindings/tests:test_interfaces",
     "//url",
   ]
 
@@ -93,5 +95,6 @@ test("mojo_common_unittests") {
     "../message_pump/handle_watcher_unittest.cc",
     "../message_pump/message_pump_mojo_unittest.cc",
     "common_type_converters_unittest.cc",
+    "service_set_unittest.cc",
   ]
 }