Add missing task_queue.cc to sync_core GN sources list.

Add missing task_queue.cc to sync_core GN sources list.

This was found by looking at the output of gyp_flag_compare step in
http://build.chromium.org/p/chromium.linux/builders/Linux%20GN/builds/25653/steps/gyp_flag_compare/logs/stdio

BUG=None
TEST=gn gen out-gn
R=brettw@chromium.org
TBR=zea@chromium.org

Committed: https://crrev.com/870780454b5e8fe4687313504b6c4d324a627df7
Cr-Commit-Position: refs/heads/master@{#322662}

[tfarina, 2015-03-26 17:20:38]

PTAL

[brettw, 2015-03-26 17:32:45]

brettw@chromium.org changed reviewers:
+ brettw@chromium.org

[brettw, 2015-03-26 17:32:45]

lgtm

[tfarina, 2015-03-27 22:16:09]

TBRing Nicolas for this, it is trivial!

[tfarina, 2015-03-27 22:16:32]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-03-27 22:17:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1038913004/1

[commit-bot: I haz the power, 2015-03-27 22:40:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-27 22:41:05]

Patchset 1 (id:??) landed as
https://crrev.com/870780454b5e8fe4687313504b6c4d324a627df7
Cr-Commit-Position: refs/heads/master@{#322662}