Remove state variables in PumpSession as they are no longer used.

This CL removes member variables of PumpSession, as they are no longer used.
Since r183572, all asynchronous parsing uses background thread and these
state variables were only used for deprecated asynchronous main thread parsing.

BUG=421189

Review URL: https://codereview.chromium.org/657753002

git-svn-id: svn://svn.chromium.org/blink/trunk@183728 bbb929c8-8fbe-4397-9dbb-9b2b20218538