bindings: Do not get if a dictionary member is missing (6/N)

bindings team is going to update IDL dictionaries to be strict about
their members' existence.  After the update, accessing a getter on a
missing dictionary member will crash.

This CL fixes the potential bug in mojo conversions in
ManifestImageResource.

Bug: 839389
Change-Id: I90d2fd5c1361345892ce972fad7727952ab5fb60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2233105Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Auto-Submit: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776841}

third_party/blink/renderer/modules/manifest/image_resource_type_converters.cc

@@ -102,9 +102,12 @@ blink::mojom::blink::ManifestImageResourcePtr TypeConverter<
                                                 image_resource) {
   auto image_resource_ptr = blink::mojom::blink::ManifestImageResource::New();
   image_resource_ptr->src = blink::KURL(image_resource->src());
-  image_resource_ptr->sizes = ParseSizes(image_resource->sizes());
-  image_resource_ptr->purpose = ParsePurpose(image_resource->purpose());
-  image_resource_ptr->type = ParseType(image_resource->type());
+  if (image_resource->hasSizes())
+    image_resource_ptr->sizes = ParseSizes(image_resource->sizes());
+  if (image_resource->hasPurpose())
+    image_resource_ptr->purpose = ParsePurpose(image_resource->purpose());
+  if (image_resource->hasType())
+    image_resource_ptr->type = ParseType(image_resource->type());
   return image_resource_ptr;
 }