Cherry-pick: Stop using HMAC for gpu mailboxes

Cherry-pick with conflict of crrev.com/r249922

Conflicts:
	gpu/command_buffer/tests/gl_texture_mailbox_unittest.cc

Original description:

For a long time, we've been using HMAC to sign the mailbox names on the GPU
process. However, we don't actually need the mailbox names to be guaranteed to
be generated by the GPU process, we only need to ensure they're unguessable.

The HMAC signature and verification has several costs:
- the cost to sign the mailbox
- the cost to verify the mailbox
- the initialization cost (e.g. starting NSS in the GPU process)

We can actually save all that cost, by just making the mailbox names
(crypto-)random.

BUG: 13328348
Change-Id: I3338a92a0908a17099d2d92de1c2bca266684fda
14 files changed