Required. The new values of feed details. It must match an existing feed
and the field name must be in the format of:
projects/project_number/feeds/feed_id or
folders/folder_number/feeds/feed_id or
organizations/organization_number/feeds/feed_id.
Required. Only updates the feed fields indicated by this mask. The field
mask must not be empty, and it must not contain fields that are immutable
or only set by the server.