Add owners for Fuchsia-specific files and directories

Added OWNERS for fuchsia dirs in //chromecast and //media. Also added
per-files OWNER rules in //net/base/OWNERS for *fuchsia*.

Change-Id: I8fbd409b6cc429dcd636c9a388e095381adaed54
Reviewed-on: https://chromium-review.googlesource.com/c/1330701
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607288}