Fix a potential null deref in XSSAuditorDelegate.

The ASAN bots say this causes a crash; I can't replicate it locally, but
I believe that the combination of `document.write` and synchronous
`javascript:` URL navigation could cause the auditor to trigger while the
document is detaching. This patch adds a small check.

BUG=668772

Review-Url: https://codereview.chromium.org/2531253002
Cr-Commit-Position: refs/heads/master@{#434927}

third_party/WebKit/Source/core/html/parser/XSSAuditorDelegate.cpp

@@ -115,7 +115,7 @@ void XSSAuditorDelegate::didBlockScript(const XSSInfo& xssInfo) {
   if (xssInfo.m_didBlockEntirePage)
     frameLoader.stopAllLoaders();
 
-  if (!m_didSendNotifications) {
+  if (!m_didSendNotifications && frameLoader.client()) {
     m_didSendNotifications = true;
 
     frameLoader.client()->didDetectXSS(m_document->url(),