Move RenderThreadImpl's GpuChannelHost logic to ui::Gpu

RenderThreadImpl's logic has a fair amount of duplication with ui::Gpu.
In particular, RenderThreadImpl shouldn't need to cache the
GpuChannelHost, since it is available directly from the ui::Gpu.

Bug: 566273
Change-Id: I6cfb9f28b46bce64c4f83aa5f2fcb4263c171018
Reviewed-on: https://chromium-review.googlesource.com/767594
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516328}
4 files changed