check if webcontents are available at tabs_api

Everywhere else access to webcontents via tab_strip->GetWebContentsAt(i)
is checked. Missing check here may lead to crash in rare cases.

R=benwells@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2839403005
Cr-Commit-Position: refs/heads/master@{#468577}

chrome/browser/extensions/api/tabs/tabs_api.cc

@@ -939,6 +939,10 @@ ExtensionFunction::ResponseAction TabsQueryFunction::Run() {
       if (index > -1 && i != index)
         continue;
 
+      if (!web_contents) {
+        continue;
+      }
+      
       if (!MatchesBool(params->query_info.highlighted.get(),
                        tab_strip->IsTabSelected(i))) {
         continue;