Add message_loop_type to GpuPreferences

Currently, the GPU process checks ui::OzonePlatform to determine the
type of message loop it should use. This means GPU process needs to
access ui::OzonePlatform instance before the instance is properly
initialized. If we check ui::OzonePlatform on the GPU host side and
send it to the GPU process via GpuPreferences, we can avoid accessing
ui::OzonePlatform before it being initialized.

BUG=958387

Change-Id: I7817cd317455f24fb55c5c8f5b8ea078a51b84b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678160
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677743}