Mus: Implement GpuMain mojo interface

The GpuMain mojo interface is the single primordial mojo interface used
by the window server to start and restart the gpu process in order to
ensure that requesting other sub-interfaces doesn't restart a crashed
GPU process and put it in a bad state.

GpuMain provides factory methods to create two sub-services: the gpu
service and the display compositor service. The gpu service does not
directly know about the display compositor service, and the display
compositor service does not know about the gpu service. GpuMain's
implementation is the glue that puts both together.

BUG=610937
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2539893002
Cr-Commit-Position: refs/heads/master@{#435509}
14 files changed