Use union type for NavigatorBeacon.sendBeacon()'s data argument

This lets us lose some overloading and code duplication.

BUG=430337

Review URL: https://codereview.chromium.org/714123002

git-svn-id: svn://svn.chromium.org/blink/trunk@185293 bbb929c8-8fbe-4397-9dbb-9b2b20218538