[Fuchsia] Add CodecFactory to cast_runner's sandbox_policy

WebEngine needs to use CodecFactory interface in order to create HW
video decoders, but CastRunner didn't have that service in the manifest,
so the coresponding web context had to use SW decoder.

Bug: Fuchsia: WEB-39
Change-Id: I9e943ef0cee06b96499992eaa1231f1021ea46b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610518
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659115}

fuchsia/runners/cast/sandbox_policy

@@ -5,6 +5,7 @@
       "fuchsia.fonts.Provider",
       "fuchsia.logger.LogSink",
       "fuchsia.media.Audio",
+      "fuchsia.mediacodec.CodecFactory",
       "fuchsia.net.SocketProvider",
       "fuchsia.netstack.Netstack",
       "fuchsia.process.Launcher",