Allow overriding of X error functions

This is a second attempt of http://codereview.chromium.org/3175038

It failed the Vista Perf UI tests. This is because those tests close the browser upon an error. And they always get an error when the session is closed in the middle of the test.

The new changes are in chrome/browser/automation/testing_automation_provider.cc

BUG=50006
TEST=Run chrome under nested window manager using Xephyr (see 
http://code.google.com/p/chromium/wiki/LayoutTestsLinux) 

use --enable-logging=stderr --log-level=0 
kill xephyr 
examine log. You should see 
X IO Error detected 
followed (not necessarily immediately) by 
successfully saved /tmp/tx/Default/Preferences 
successfully saved /tmp/tx/Local State 
successfully saved /tmp/tx/Local State 
successfully saved /tmp/tx/Default/Preferences 
along with no crash. 



BUG=
TEST=

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=59269

[darin (slow to review), 2010-09-13 18:37:07]

http://codereview.chromium.org/3364019/diff/1/7
File chrome/browser/automation/testing_automation_provider.cc (right):

http://codereview.chromium.org/3364019/diff/1/7#newcode423
chrome/browser/automation/testing_automation_provider.cc:423: if
(browser_shutdown::GetShutdownType() == browser_shutdown::NOT_VALID)
LGTM

This might be clearer if we had a helper function named
browser_shutdown::IsShuttingDown()