Drop PromiseResolver object from internal Promises implementation

Since PromiseResolver is no longer needed, we drop it from the internal
implementation of Promises. To follow the concept of AP2 consensus[1],
rename "PendingWithResolvedFlagSet" to "Follow".

[1]: https://github.com/domenic/promises-unwrapping

BUG=295420

Review URL: https://chromiumcodereview.appspot.com/23714010

git-svn-id: svn://svn.chromium.org/blink/trunk@158149 bbb929c8-8fbe-4397-9dbb-9b2b20218538