PPB_VideoDecoder: create mailbox names on the plugin side

For obscure historical reasons, mailbox names were created on the
renderer side whereas the textures are produced on the plugin side. Move
the mailbox name creation to the plugin side so that it happens when the
textures are produced. A follow-up will merge the 2 operations.

Bug: 847674
Change-Id: Ia8cde57c2c0421e1361ddb14de63f02a34ea9b6f
Reviewed-on: https://chromium-review.googlesource.com/1081669
Reviewed-by: Bill Budge <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Commit-Queue: Antoine Labour <[email protected]>
Cr-Commit-Position: refs/heads/master@{#563858}
8 files changed