Check if web_contents are valid before sending SB hit report

This CL is to fix the crash caused by invalid web_contents access in
safe_browsing::SafeBrowsingUIManager::MaybeReportSafeBrowsingHit

Bug: 888159
Change-Id: I6e7beaa5cd4d7c249d03465e3924635bead0d283
Reviewed-on: https://chromium-review.googlesource.com/1241314
Commit-Queue: Jialiu Lin <jialiul@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593709}

chrome/browser/safe_browsing/ui_manager.cc

@@ -108,7 +108,8 @@ void SafeBrowsingUIManager::ShowBlockingPageForResource(
 bool SafeBrowsingUIManager::ShouldSendHitReport(
     const HitReport& hit_report,
     const WebContents* web_contents) {
-  return hit_report.extended_reporting_level != SBER_LEVEL_OFF &&
+  return web_contents &&
+         hit_report.extended_reporting_level != SBER_LEVEL_OFF &&
          !web_contents->GetBrowserContext()->IsOffTheRecord();
 }