add dcastagna@ to //media/gpu OWNERS

Proposing dcastagna@ to join the //media/gpu OWNERS.

The file is sorted alphabetically.

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ia96664905ec5cfbcd3879bcb7006b15db0ae6aeb
Reviewed-on: https://chromium-review.googlesource.com/956287Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541908}

media/gpu/OWNERS

 dalecurtis@chromium.org
+dcastagna@chromium.org
+jcliang@chromium.org
 kcwu@chromium.org
 posciak@chromium.org
 sandersd@chromium.org
 wuchengli@chromium.org
-jcliang@chromium.org
 
 # For Android media gpu files.
 per-file *android*=liberato@chromium.org