This adds a path for VDAv2 clients to look up a command buffer identified by |command_buffer_id|.

Also in this CL is a method to look up the route ID from the accelerated GPU factories so that it's easy to pass the |command_buffer_id|.

BUG=522298
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/2526953003
Cr-Commit-Position: refs/heads/master@{#443097}