Add _fuchsia wildcard OWNERS for media/ and mojo/

Bug: 
Change-Id: I3c80fa4240da38bf1ff6a24c89d5501b9d7e27ed
Reviewed-on: https://chromium-review.googlesource.com/646728
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#500775}

diff --git a/media/OWNERS b/media/OWNERS
index 50e4f03..d8221ce 100644
--- a/media/OWNERS
+++ b/media/OWNERS

@@ -20,4 +20,7 @@
 wolenetz@chromium.org
 xhwang@chromium.org
 
+# For Fuchsia-specific changes:
+per-file *_fuchsia*=file://build/fuchsia/OWNERS
+
 # COMPONENT: Internals>Media