Turn on the fixed position stacking context flag by default

Because there's an expectation that this flag will need to be reverted on a
branch, add both the enable and the disable flags so that only
web_contents_impl.cc will need to be touched to change the default value.
Both can be removed later, once this setting sticks.

BUG=none
TEST=none


Review URL: https://chromiumcodereview.appspot.com/10825027

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148572 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 7fd47682..5b5d4bde 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -597,8 +597,8 @@
     prefs.apply_default_device_scale_factor_in_compositor = true;
   }
 
-  prefs.fixed_position_creates_stacking_context =
-      command_line.HasSwitch(switches::kFixedPositionCreatesStackingContext);
+  prefs.fixed_position_creates_stacking_context = !command_line.HasSwitch(
+      switches::kDisableFixedPositionCreatesStackingContext);
 
   prefs.number_of_cpu_cores = base::SysInfo::NumberOfProcessors();