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This is needed for the call to base::PostTaskWithTraits in
ui::DrmDisplayHostManager::ProcessEvent.

BUG=678598
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/2620653003
Cr-Commit-Position: refs/heads/master@{#442898}