Mojo C++ bindings: support mapping T and T? differently.

components/arc/test/fake_app_instance.cc

Index: components/arc/test/fake_app_instance.cc
diff --git a/components/arc/test/fake_app_instance.cc b/components/arc/test/fake_app_instance.cc
index 64822b753a92bead33f4e07fe6eb441e5bb39514..8e734a6fb06d76905fc88b928efeb993e634d8ca 100644
--- a/components/arc/test/fake_app_instance.cc
+++ b/components/arc/test/fake_app_instance.cc
@@ -47,7 +47,7 @@ void FakeAppInstance::RefreshAppList() {
 
 void FakeAppInstance::LaunchApp(const mojo::String& package_name,
                                 const mojo::String& activity,
-                                const gfx::Rect& dimension) {
+                                const base::Optional<gfx::Rect>& dimension) {
   launch_requests_.push_back(new Request(package_name, activity));
 }
 
@@ -207,8 +207,9 @@ void FakeAppInstance::SetNotificationsEnabled(const mojo::String& package_name,
 
 void FakeAppInstance::InstallPackage(mojom::ArcPackageInfoPtr arcPackageInfo) {}
 
-void FakeAppInstance::LaunchIntent(const mojo::String& intent_uri,
-                                   const gfx::Rect& dimension_on_screen) {
+void FakeAppInstance::LaunchIntent(
+    const mojo::String& intent_uri,
+    const base::Optional<gfx::Rect>& dimension_on_screen) {
   launch_intents_.push_back(new mojo::String(intent_uri));
 }