Summary:
changelog: [internal]
In this diff, we delete default initialised for ShadowViewMutation to prevent accidentally creating empty ShadowViewMutation.
The other initialiser is made private and all of its uses are migrated to designated initialisers. This makes for safer API.
Reviewed By: RSNara
Differential Revision: D30774900
fbshipit-source-id: d2064bf08409850e75e13ad06558b7980a7f5d8d