[getUserMedia] Change frame reference from WeakMember to Member.

Also perform an extra null check when the frame is converted to a
WebFrame.

Bug: 1017121
Change-Id: I2e85f8db9917076a6865b4411e69dc81230764c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876410Reviewed-by: Armando Miraglia <armax@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708643}

third_party/blink/renderer/modules/mediastream/user_media_processor.cc

@@ -849,9 +849,7 @@ UserMediaProcessor::GetMediaStreamDeviceObserver() {
       media_stream_device_observer_for_testing_;
   if (frame_) {  // Can be null for tests.
     auto* web_frame = static_cast<WebLocalFrame*>(WebFrame::FromFrame(frame_));
-    DCHECK(web_frame);
-
-    if (!web_frame->Client())
+    if (!web_frame || !web_frame->Client())
       return nullptr;
 
     // TODO(704136): Move ownership of |WebMediaStreamDeviceObserver| out of

third_party/blink/renderer/modules/mediastream/user_media_processor.h

@@ -294,7 +294,7 @@ class MODULES_EXPORT UserMediaProcessor
   MediaDevicesDispatcherCallback media_devices_dispatcher_cb_;
   base::OnceClosure request_completed_cb_;
 
-  WeakMember<LocalFrame> frame_;
+  Member<LocalFrame> frame_;
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
   THREAD_CHECKER(thread_checker_);