Remove the compile-time selection of the GL implementation based on the

Remove the compile-time selection of the GL implementation based on the
GR_SUPPORT_GLDESKTOP family of macros.

Support for the platform is configured dynamically, by querying the
fBindingsExported member of active GrGLInterface instance.

Review: http://codereview.appspot.com/4298048/

git-svn-id: http://skia.googlecode.com/svn/trunk@960 2bbb7eff-a529-9590-31e7-b0007b416f81