bindings: Check if a dictionary member is not missing before accessing

This CL adds checking if |config->profile| is available before
accessing it.

Bug: 839389
Change-Id: I92214e32575f86c899c4a6fc3aeea189877118d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358941
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798553}

third_party/blink/renderer/modules/webcodecs/video_encoder.cc

@@ -289,7 +289,9 @@ void VideoEncoder::ProcessConfigure(Request* request) {
   }
 
   std::string codec_str = config->codec().Utf8();
-  std::string profile_str = config->profile().Utf8();
+  std::string profile_str;
+  if (config->hasProfile())
+    profile_str = config->profile().Utf8();
   auto codec_type = media::StringToVideoCodec(codec_str);
   if (codec_type == media::kUnknownVideoCodec) {
     HandleError(DOMExceptionCode::kNotFoundError, "Unknown codec type");