delete useThreading flag as it is always true

Since r183572, the flag is always true.
This CL removes the flag and dead codes referencing the flag.

BUG=421289

Review URL: https://codereview.chromium.org/657453003

git-svn-id: svn://svn.chromium.org/blink/trunk@183715 bbb929c8-8fbe-4397-9dbb-9b2b20218538