Fix crash when an extension popup shows a JS alert. Showing the alert takes...

Fix crash when an extension popup shows a JS alert. Showing the alert takes...

Landing this on for pam. Original issue: http://codereview.chromium.org/425011/show.

Disable showing JS alerts from popups, because doing so makes the popup 
disappear immediately, which has all sorts of unfortunate side effects for the 
poor orphaned alert (see bug for details). 

BUG=27758
TEST=create extension with popup, put link with |onclick="alert('test');"| in 
it; install extension, open popup, and click link; see no crash (nor popup)

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=32889