Remove SynchronousMode, as it is always ForceSynchronous.

Since r183572, SynchronousMode is alywas ForceSynchronous, as all
asynchronous parsing now uses background thread.

This CL removes the enum and removes dead codes.

BUG=421289

Review URL: https://codereview.chromium.org/659713002

git-svn-id: svn://svn.chromium.org/blink/trunk@183725 bbb929c8-8fbe-4397-9dbb-9b2b20218538