Allow constructing a StrongBinding with an InterfacePtr*

Binding<> allows this, StrongBinding should as well.

R=davemoore@chromium.org

Review URL: https://codereview.chromium.org/721123004

mojo/public/cpp/bindings/strong_binding.h

diff --git a/mojo/public/cpp/bindings/strong_binding.h b/mojo/public/cpp/bindings/strong_binding.h
index 73966ff..106b282 100644
--- a/mojo/public/cpp/bindings/strong_binding.h
+++ b/mojo/public/cpp/bindings/strong_binding.h
@@ -51,6 +51,14 @@
 
   StrongBinding(
       Interface* impl,
+      InterfacePtr<Interface>* ptr,
+      const MojoAsyncWaiter* waiter = Environment::GetDefaultAsyncWaiter())
+      : StrongBinding(impl) {
+    binding_.Bind(ptr, waiter);
+  }
+
+  StrongBinding(
+      Interface* impl,
       InterfaceRequest<Interface> request,
       const MojoAsyncWaiter* waiter = Environment::GetDefaultAsyncWaiter())
       : StrongBinding(impl) {