browser test startup timeout from 10s to 20s

10s is on the edge of reliable on my macbookpro.
it's mostly unreliable on the autobuild machine.
raising to 20s.

[kpreid2, 2013-09-13 00:31:22]

LGTM

Do we know what got slower?

[felix8a, 2013-09-13 00:45:48]

On 2013/09/13 00:31:22, kpreid2 wrote:
> LGTM
> 
> Do we know what got slower?

I don't think anything got slower, I was just over-optimistic when removing the
async test firing. The async test firing was largely motivated by one test that
creates about a dozen caja guest frames before anything makes progress. In
retrospect, async firing was unnecessary complexity and fragility for just that
one problem.

[felix8a, 2013-09-13 00:46:26]

@r5596