Fix browser crash due to bookmark bubble view loosing activation while gettin...

Fix browser crash due to bookmark bubble view losing activation while getting shown.

As mentioned in the comment:
|bubble_| can be set to NULL in InfoBubbleClosing when we close the bubble
asynchronously. However, that can happen during the Show call above if the
window loses activation while we are getting to ready to show the bubble,
so we must check to make sure we still have a valid bubble before
proceeding.


BUG=59524
TEST=I tried creating a UI test for this, but wasn't able to reproduce the crash with
a test.


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=71028

[sky, 2011-01-10 17:37:26]

LGTM