Let SkTaskGroup work synchronously if no one created an SkTaskGroup::Enabler.

Let SkTaskGroup work synchronously if no one created an SkTaskGroup::Enabler.

Tested by running DM with its SkTaskGroup::Enabler commented out.  Slow, but completed correctly.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/eefd18cef26dcfb67e68d05e1845b35f015a5bca

[mtklein, 2014-09-12 16:28:09]

mtklein@google.com changed reviewers:
+ mtklein@google.com, reed@google.com

[mtklein, 2014-09-12 16:28:12]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-09-12 16:28:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/556223003/1

[commit-bot: I haz the power, 2014-09-12 16:28:24]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from a valid reviewer till 2014-09-12 22:28 UTC

[reed1, 2014-09-12 16:29:24]

lgtm

[commit-bot: I haz the power, 2014-09-12 16:35:59]

Committed patchset #1 (id:1) as eefd18cef26dcfb67e68d05e1845b35f015a5bca