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GpuChildThread itself never uses |gpu_preferences_|. However,
GpuChannelManager holds a reference to it. So remove it from
GpuChildThread, and have GpuChannelManager, which actually uses
it, own it instead.

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/2322893002
Cr-Commit-Position: refs/heads/master@{#417331}