GpuWatchdogThread doesn't have any dependency on content. So move it into
the gpu service component, so that it can be used by both content-gpu and
mus-gpu.
BUG=none
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Review-Url: https://codereview.chromium.org/2286063003
Cr-Commit-Position: refs/heads/master@{#415149}