Fix a crash in WebRequestInternalEventHandledFunction::Run

Dereferencing a null extension(), change it to use extension_id_safe()

bug: chromium:795399
Change-Id: If072049509dca199707c48ff374c6a296b45625a
Reviewed-on: https://chromium-review.googlesource.com/831186Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Ivan Šandrk <isandrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524603}

extensions/browser/api/web_request/web_request_api.cc

@@ -2095,7 +2095,7 @@ WebRequestInternalEventHandledFunction::Run() {
     // In Public Session we only want to allow "cancel" (except for whitelisted
     // extensions which have no such restrictions).
     if (IsPublicSession() &&
-        !extensions::IsWhitelistedForPublicSession(extension()->id()) &&
+        !extensions::IsWhitelistedForPublicSession(extension_id_safe()) &&
         (value->HasKey("redirectUrl") ||
          value->HasKey(keys::kAuthCredentialsKey) ||
          value->HasKey("requestHeaders") ||