Use UseProtocolCachePolicy for subresources when loading the main resource as type FrameLoadTypeSame

Testing: Follow the steps in https://crbug.com/393649#c9

BUG=393649

Review URL: https://codereview.chromium.org/404903002

git-svn-id: svn://svn.chromium.org/blink/trunk@180716 bbb929c8-8fbe-4397-9dbb-9b2b20218538