Check web_contents before getting navigation ID

When password protection verdict comes back from Safe Browsing backend,
it is possible that the web_contents has already gone. Therefore, we
need to check if web_contents is still valid before getting navigation
ID from it.

Bug: 885359
Change-Id: I3c80a8f2a3158ad3c459317e4d4954aa60f6d1f9
Reviewed-on: https://chromium-review.googlesource.com/1236816Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Jialiu Lin <jialiul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592992}

chrome/browser/safe_browsing/chrome_password_protection_service.cc

@@ -115,6 +115,8 @@ PasswordReuseLookup::ReputationVerdict GetVerdictToLogFromResponse(
 // Given a |web_contents|, returns the navigation id of its last committed
 // navigation.
 int64_t GetLastCommittedNavigationID(const content::WebContents* web_contents) {
+  if (!web_contents)
+    return 0;
   content::NavigationEntry* navigation =
       web_contents->GetController().GetLastCommittedEntry();
   return navigation