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The file-descriptors in NativePixmapHandle need to be dup'ed before then
can be sent over mojo IPC from InProcessCommandBuffer. Introduce a
convenience function CloneHandleForIPC() so that it can be used by both
GpuChannelHost and InProcessCommandBuffer.

BUG=643746
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/2531963002
Cr-Commit-Position: refs/heads/master@{#434884}