Add some tests for ThrottlingResourceHandler.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 7622839031c3315bbe5808b3d0547cc41fb3f3a5..1511bfb9ce7cb0b04d375c342ea21e649594b2e3 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -1100,6 +1100,8 @@ test("content_unittests") {
     "../browser/loader/async_revalidation_manager_unittest.cc",
     "../browser/loader/intercepting_resource_handler_unittest.cc",
     "../browser/loader/mime_sniffing_resource_handler_unittest.cc",
+    "../browser/loader/mock_resource_loader.cc",
+    "../browser/loader/mock_resource_loader.h",
     "../browser/loader/mojo_async_resource_handler_unittest.cc",
     "../browser/loader/navigation_url_loader_unittest.cc",
     "../browser/loader/resource_buffer_unittest.cc",
@@ -1111,6 +1113,7 @@ test("content_unittests") {
     "../browser/loader/test_resource_handler.h",
     "../browser/loader/test_url_loader_client.cc",
     "../browser/loader/test_url_loader_client.h",
+    "../browser/loader/throttling_resource_handler_unittest.cc",
     "../browser/loader/upload_data_stream_builder_unittest.cc",
     "../browser/loader/url_loader_factory_impl_unittest.cc",
     "../browser/mach_broker_mac_unittest.cc",