Implement Storage Id for encrypted media

Adds code to compute Storage Id for encrypted media. It uses a new profile
setting ("media.storage_id_salt"). On Chromium this is not supported, so the
empty string is returned. Chrome implementation pending.

BUG=478960
TEST=StorageId browser_tests pass

Change-Id: I566e1cfbd1d44a2be809dd51c8418b922126cccc
Reviewed-on: https://chromium-review.googlesource.com/587828
Reviewed-by: Brett Wilson <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Bill Budge <[email protected]>
Reviewed-by: Thomas Guilbert <[email protected]>
Reviewed-by: Bernhard Bauer <[email protected]>
Commit-Queue: John Rummell <[email protected]>
Cr-Commit-Position: refs/heads/master@{#498600}
17 files changed