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To remove duplicate code. In another CL, I also need it in ClearKeyCdm.

Bug: 169203
Test: No functionality change.
Change-Id: I077d0b52780e960f4da6e974405eff6a2dd2fe06
Reviewed-on: https://chromium-review.googlesource.com/1247561Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594563}