Fix leak of PlatformHandleDispatchers in Mojo IPC

PassWrappedPlatformHandle does not close the MojoHandle.
The destructor of ScopedHandleBase will however close it.
The problem with this code was that it called release()
on the ScopedHandle, preventing the destructor from being
called, and thus also the MojoHandle from being closed.

This leaves the PlatformHandleDispatcher associated with
the MojoHandle alive in mojo::system::Core::handle_table_

By calling reset() instead the MojoHandle is closed, and
the leak is fixed.

R=morrita@chromium.org, viettrungluu@chromium.org, agl@chromium.org

BUG=None

Review URL: https://codereview.chromium.org/1174423002

Cr-Commit-Position: refs/heads/master@{#334197}

ipc/mojo/ipc_mojo_handle_attachment.cc

@@ -25,7 +25,8 @@ MessageAttachment::Type MojoHandleAttachment::GetType() const {
 base::PlatformFile MojoHandleAttachment::TakePlatformFile() {
   mojo::embedder::ScopedPlatformHandle platform_handle;
   MojoResult unwrap_result = mojo::embedder::PassWrappedPlatformHandle(
-      handle_.release().value(), &platform_handle);
+      handle_.get().value(), &platform_handle);
+  handle_.reset();
   if (unwrap_result != MOJO_RESULT_OK) {
     LOG(ERROR) << "Pipe failed to covert handles. Closing: " << unwrap_result;
     return -1;