Attempt to address libxml crash.

BUG=129930
Review URL: https://chromiumcodereview.appspot.com/10458051

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@142822 0039d316-1c4b-4281-b951-d872f2087c98
xmlThrDefSetStructuredErrorFunc(void *ctx, xmlStructuredErrorFunc handler) {
    xmlMutexLock(xmlThrDefMutex);
    xmlStructuredErrorContextThrDef = ctx;
    xmlStructuredErrorThrDef = handler;
    xmlMutexUnlock(xmlThrDefMutex);
}

xmlThrDefSetStructuredErrorFunc(void *ctx, xmlStructuredErrorFunc handler) {
    xmlMutexLock(xmlThrDefMutex);
    xmlStructuredErrorContextThrDef = ctx;
    xmlStructuredErrorThrDef = handler;
    xmlMutexUnlock(xmlThrDefMutex);
}
