Fix media error mojom serialization

There were no checks in the StructTraits static getter methods, which
was causing DCHECK's when used.

Bug: 1043289
Change-Id: Ic139687493bb1e03b369dda2f30301ee78aa8330
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067949Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745141}

media/mojo/mojom/media_error_mojom_traits.h

@@ -20,15 +20,24 @@ struct StructTraits<media::mojom::MediaErrorDataView, media::MediaError> {
     return input.GetErrorCode();
   }
 
-  static const std::string& message(const media::MediaError& input) {
+  static base::Optional<std::string> message(const media::MediaError& input) {
+    if (input.IsOk())
+      return base::nullopt;
+    DCHECK(input.data_);
     return input.GetErrorMessage();
   }
 
   static base::span<base::Value> frames(const media::MediaError& input) {
+    if (input.IsOk())
+      return {};
+    DCHECK(input.data_);
     return input.data_->frames;
   }
 
   static base::span<media::MediaError> causes(const media::MediaError& input) {
+    if (input.IsOk())
+      return {};
+    DCHECK(input.data_);
     return input.data_->causes;
   }