Add top-level gpu OWNERS

We need more OWNERS... move up some command_buffer OWNERS, and add kbr@

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/2348773002
Cr-Commit-Position: refs/heads/master@{#419080}

gpu/OWNERS

+jbauman@chromium.org
+kbr@chromium.org
 piman@chromium.org
 sievers@chromium.org
+vmiura@chromium.org
+zmo@chromium.org
 
 # GPU memory buffer implementations.
 per-file *gpu_memory_buffer*=reveman@chromium.org