Reduce resolution of performance.now to prevent timing attacks

Reduce resolution of performance.now to prevent timing attacks

This is a backport of the following WebKit patches by
Alex Christensen <achristensen@apple.com>;:
http://trac.webkit.org/changeset/186208
http://trac.webkit.org/changeset/186219

BUG=506723
TEST=http/tests/misc/webtiming-resolution.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=198348

[Kunihiko Sakamoto, 2015-07-06 05:55:15]

ksakamoto@chromium.org changed reviewers:
+ pdr@chromium.org

[pdr., 2015-07-06 22:18:47]

pdr@chromium.org changed reviewers:
+ inferno@chromium.org, parisa@chromium.org

[pdr., 2015-07-06 22:18:47]

This is correct but I'd prefer if someone from the security team could give this
the final OK. Is this the best mitigation against that paper?

+cc the coolest security engineers.

[inferno, 2015-07-06 22:27:33]

inferno@chromium.org changed reviewers:
+ mseaborn@chromium.org
- parisa@chromium.org

[inferno, 2015-07-06 22:32:33]

On 2015/07/06 22:18:47, pdr wrote:
> This is correct but I'd prefer if someone from the security team could give
this
> the final OK. Is this the best mitigation against that paper?
> 
> +cc the coolest security engineers.

lgtm based on research response in email -  "Our attack as described becomes
very difficult to launch once the resolution of performance.now() is
upper-bounded to 5 thousandths of a millisecond."

[pdr., 2015-07-06 22:55:24]

LGTM for owner approval.

[pdr., 2015-07-06 22:55:26]

The CQ bit was checked by pdr@chromium.org

[commit-bot: I haz the power, 2015-07-06 22:55:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1214063008/1

[commit-bot: I haz the power, 2015-07-06 23:47:58]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=198348