SMS Receiver: Fix crash in sms due to |frame| == nullptr.

This change add a check to ensure frame is available, and to return
and error if not before proceeding with the call.

Bug: 1027394
Change-Id: Ib839321b7a9eef193b4285523ac60344d4a3a049
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1947192
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721763}

third_party/blink/renderer/modules/sms/sms_receiver.cc

@@ -36,6 +36,12 @@ ScriptPromise SMSReceiver::receive(ScriptState* script_state,
   DCHECK(context->IsContextThread());
 
   LocalFrame* frame = GetFrame();
+  if (!frame) {
+    exception_state.ThrowDOMException(DOMExceptionCode::kNotSupportedError,
+                                      "Script context has shut down.");
+    return ScriptPromise();
+  }
+
   if (!frame->IsMainFrame() && frame->IsCrossOriginSubframe()) {
     exception_state.ThrowDOMException(
         DOMExceptionCode::kNotAllowedError,