GitLab

Add code so that media encrypted in 'cbcs' encryption scheme can be
handled by AesDecryptor.

This adds 2 new classes:
  aes_cbc_crypto.* handles AES-CBC-128 decryption using the BoringSSL
    code to allow for decryption without padding and to avoid extra copies
    of the data.
  cbcs_decryption.* supports pattern decryption.
Both classes have unit tests to verify their basic functionality.

BUG=657957,658026
TEST=content_browsertests pass

Change-Id: Ia0a8db2859ff4f815a0cfcc705f7ea50d77a115c
Reviewed-on: https://chromium-review.googlesource.com/1038725
Commit-Queue: John Rummell <jrummell@chromium.org>
Reviewed-by: Kongqun Yang <kqyang@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557127}