Adds ScopedHandleBase::From

mojo/public/cpp/system/core.h

Index: mojo/public/cpp/system/core.h
diff --git a/mojo/public/cpp/system/core.h b/mojo/public/cpp/system/core.h
index 70221107ab0a2d6ee332d461294840c34c601d55..d7be3f3f8d454c9b8d08b6981deb916c8bd79a95 100644
--- a/mojo/public/cpp/system/core.h
+++ b/mojo/public/cpp/system/core.h
@@ -49,6 +49,16 @@ class ScopedHandleBase {
 
   const HandleType& get() const { return handle_; }
 
+  template <typename PassedHandleType>
+  static ScopedHandleBase<HandleType> From(
+      ScopedHandleBase<PassedHandleType> other) {
+    MOJO_COMPILE_ASSERT(
+        sizeof(static_cast<PassedHandleType*>(static_cast<HandleType*>(0))),
+        PassedHandleType_is_not_a_subtype_of_HandleType);

[viettrungluu, 2014/04/03 21:19:08]

This should be the other way around:
HandleType_is_not_a_subtype_of_PassedHandleType.

[sky, 2014/04/03 21:26:45]

Done.

+    return ScopedHandleBase<HandleType>(
+        static_cast<HandleType>(other.release().value()));
+  }
+
   void swap(ScopedHandleBase& other) {
     handle_.swap(other.handle_);
   }