Fix syncing of system theme on Linux

Fix syncing of system theme on Linux

Makes the "IsSystemThemeDistinctFromDefaultTheme()" detection logic
compatible with non-GTK (ie. Aura) Linux.  It moves the check out of
sync code and into the ThemeService so this kind of bug will be less
likely to recur if the themes logic changes again.

Re-enables an integration test that was disabled during the transition
to Aura.

BUG=304554, 392934
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282491

[rlarocque, 2014-07-10 20:48:07]

haitaol: Please review theme_syncable_service.cc and
two_client_themes_sync_test.cc
pkotwicz: Please review chrome/browser/themes/*

[erg, 2014-07-10 21:14:38]

(fwiw, lgtm, and thank you for tracking this down.)

[haitaol1, 2014-07-10 21:18:13]

lgtm

[pkotwicz, 2014-07-10 21:25:46]

LGTM. Thanks for fixing this!

[rlarocque, 2014-07-10 21:26:41]

The CQ bit was checked by rlarocque@chromium.org

[commit-bot: I haz the power, 2014-07-10 21:27:44]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rlarocque@chromium.org/388563004/1

[commit-bot: I haz the power, 2014-07-10 22:50:24]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_gn_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_gn_rel...)
  mac_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/mac_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-07-11 01:20:14]

Change committed as 282491