cc: Remove racy initialization code in cc/base/switches.cc.

This moves all lazy initialization code out of cc/base/switches.cc. Code
that needs the value of these switches in a critical path should not rely
on lazy initialization in cc/switches.cc but instead handle this on its own.

Thread-safe lazy initialization code of impl-side painting status has
been added to WebLayerImpl.

This also removes the cc::switches::IsGPURasterizationEnabled() function,
which for consistency should not exist unless the logic to determine the
value is more complicated than HasSwitch(kEnableGPURasterization). This
function was never used in performance sensitive code so lazy initialization
was never necessary.

BUG=330937
TBR=jamesr

Review URL: https://codereview.chromium.org/122063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243432 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243432