Fix flakiness in unload browser test, take two. I was able to repro this on my Linux box (~10% of t…

Fix flakiness in unload browser test, take two. I was able to repro this on my Linux box (~10% of the time). The issue turned out to be that the beforeunload ack from the renderer would sometimes come after the second time that we try to close the tab. The browser wouldn't put up the beforeunload prompt because it was waiting for the previous ack. The old ui test had a 10 second sleep here which masked this.

BUG=123110
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=132217

[eroman, 2012-04-13 18:31:07]

lgtm