Fix memory leak in extensions::api::SerialGetDevicesFunction

This change resets the SerialDeviceEnumeratorPtr held by this class
after enumeration is complete. This breaks the reference cycle between
these two objects, allowing them both to be freed.

Bug: 898177
Change-Id: I874c62f89f8bb59a2c41f8c40afd537404f4580b
Reviewed-on: https://chromium-review.googlesource.com/c/1311533
Reviewed-by: Ken Rockot <[email protected]>
Commit-Queue: Reilly Grant <[email protected]>
Cr-Commit-Position: refs/heads/master@{#604415}
1 file changed