Adding application name message in kiosk mode after user timeout

This is a security feature for a user who interacts with a kiosk device - making sure that he/she knows which application is running.

When a user interacts with the device after 2 minutes of inactivity, a message shows which application is running and who was the author.

When spoken feedback is turned on, the device will also narrowate the content of the message.

The message itself will slide in from the top and - after a short timeout - will automatically slide out again.

BUG=268453
TEST=unit test & visual test

Review URL: https://codereview.chromium.org/131353007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249921 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249921
Stop using HMAC for gpu mailboxes

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=311259

Review URL: https://codereview.chromium.org/157243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249922 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249922