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This class can be useful for any classes to implement/support multiple
versins of library CDM interface. Hence, move it out of clear_key_cdm
folder. In a later CL, I'll use them for a mock library CDM
implementation.

Bug: 836046
Test: No functionality change.
Change-Id: I7567e405de0251ddce3718eb76d102f07ff8f33e
Reviewed-on: https://chromium-review.googlesource.com/1026970Reviewed-by: John Rummell <jrummell@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553693}