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 7cbd5bf40c6358a63b8f86a8b935db34d747a6c9..2a6d56920990e03c705769590f8b17219b0222a0 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",
+    "strong_binding_set.h",
     "user_agent.cc",
     "user_agent.h",
     "weak_binding_set.h",
@@ -84,6 +85,7 @@ test("mojo_common_unittests") {
     "//mojo/message_pump",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/test_support:test_utils",
+    "//mojo/public/interfaces/bindings/tests:test_interfaces",
     "//testing/gtest",
     "//third_party/mojo/src/mojo/edk/test:run_all_unittests",
     "//third_party/mojo/src/mojo/edk/test:test_support",
@@ -96,6 +98,7 @@ test("mojo_common_unittests") {
     "../message_pump/handle_watcher_unittest.cc",
     "../message_pump/message_pump_mojo_unittest.cc",
     "common_type_converters_unittest.cc",
+    "strong_binding_set_unittest.cc",
   ]
 }