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Before this CL, MockAgentSchedulingGroup would always invoke the
AgentSchedulingGroup base class constructor which is now reserved for
the kLegacy MBI feature mode. This caused the ctor to DCHECK in tests
when MBI mode was enabled.

After this CL, MockAgentSchedulingGroup conditionally invokes the right
AgentSchedulingGroup constructor given the flag. Consequently,
MockRenderThread::GetIOTaskRunner() needs to be able to return a
non-null TaskRunner. This CL gives MockRenderThread the ability to set
a new IOTaskRunner manually so that it can be returned when necessary.

While we're here we also remove
PrintMockRenderThread_set_io_thread_for_testing() since the base class
MockRenderThread now handles that. We also entirely remove
ChromeMockRenderThread.

Bug: 1158656
Change-Id: I1ab1231048ecc1b798549634811706f86a7bafb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2591929
Commit-Queue: Dominic Farolino <dom@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837106}