Decouple media object creation from IPC

Background: We would like to transition to a state where
Media services live in a layer above Gpu, and content or
mus live in a layer above both.

Practically, what the likely end state will be is a mojo
command buffer interface, and associated mojo media
(encode/decode) interfaces. The command buffer interface
will dispense media interfaces. In order to faciliate this
transition, we should minimize exposing IPC details across
layers.

Thus, this CL removes IPC details going from Gpu to Media.

BUG=586384

Review URL: https://codereview.chromium.org/1749393002

Cr-Commit-Position: refs/heads/master@{#378672}
9 files changed