Rather than allocating a buffer directly, add CdmAllocator interface
that will allocate buffers as needed. Initial implementation of
CdmAllocator (SimpleCdmAllocator) simply creates a new buffer every
time one is needed. There is no reuse, since it will only be used
for testing purposes.

CdmAllocator also has an interface to create a cdm::VideoFrame,
which now has the additional ability to create a media::VideoFrame
using the data returned by the CDM.

BUG=510088
TEST=Updated tests pass

Review URL: https://codereview.chromium.org/1673383002

Cr-Commit-Position: refs/heads/master@{#375987}