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It is currently just an alias to MessagePump::Type. This patch changes
all references to use MessagePump::Type as MessageLoop will eventually
disappear.

This patch also changes some variable names from message_loop_type to
message_pump_type. In particular one field of the GpuPreferences mojo
struct. It is weird to see things like

message_loop_type = MessagePump::Type::UI

BUG=891670

Change-Id: I44680b19c83f009f7fbb0900f4d5b1140d51457f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726050Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#682635}