delete useThreading flag as it is always true

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183715

[kouhei (in TOK), 2014-10-15 01:09:56]

kouhei@chromium.org changed reviewers:
+ eseidel@chromium.org, haraken@chromium.org

[haraken, 2014-10-15 01:10:59]

LGTM

[kouhei (in TOK), 2014-10-15 01:11:39]

The CQ bit was checked by kouhei@chromium.org

[commit-bot: I haz the power, 2014-10-15 01:12:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/657453003/1

[commit-bot: I haz the power, 2014-10-15 01:12:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/657453003/1

[commit-bot: I haz the power, 2014-10-15 01:35:08]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-10-15 01:35:09]

Try jobs failed on following builders:
  linux_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_rel/builds/2...)

[kouhei (in TOK), 2014-10-15 02:00:58]

The CQ bit was checked by kouhei@chromium.org

[commit-bot: I haz the power, 2014-10-15 02:01:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/657453003/20001

[commit-bot: I haz the power, 2014-10-15 04:01:45]

Committed patchset #2 (id:20001) as 183715