mojo: Don't leak handles when assigning to ScopedHandles.

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 2b1caa914a4b69770a9e632dbad6e01564bb5229..13be3eb3f746c39d47c67cbca0538efbf5ba90e2 100644
--- a/mojo/public/cpp/system/core.h
+++ b/mojo/public/cpp/system/core.h
@@ -107,7 +107,10 @@ class ScopedHandleBase {
   // Move-only constructor and operator=.
   ScopedHandleBase(RValue other) : handle_(other.object->release()) {}
   ScopedHandleBase& operator=(RValue other) {
-    handle_ = other.object->release();
+    if (other.object != this) {
+      CloseIfNecessary();
+      handle_ = other.object->release();
+    }
     return *this;
   }