Connect WebGPUDecoder to WireServer interfaces for handling Dawn commands

This adds a DawnCommands command which lets the WebGPUDecoder receive
and handle a block of Dawn commands via shared memory. The decoder
serializes return commands into a buffer and sends it to the client by
calling DecoderClient::HandleReturnData.

Bug: 877147
Change-Id: Ie02e67d8debdb093b24701a29de706fbc0533fe7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1481616
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639755}
11 files changed