Having a single global API and Driver object doesn't mirror the real GL
context state that may have different entry points, extensions and version
numbers per context.  This can cause entry points to be called when they
are not loaded or extensions used when they are not present.

 * Refactored the GLApi and DriverGL objects to be owned by GLContext
   objects.
 * Moved all driver-level GL workarounds into RealGLApi so no swapping of
   function pointers in DriverGL is needed.
 * Instead of having separate debug function pointers in the drivers that
   call back to global variables, add new DebugApi objects that wrap the
   RealApi objects.

BUG=671217
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
R=jbauman@chromium.org

Review-Url: https://codereview.chromium.org/2629633003 .
Cr-Commit-Position: refs/heads/master@{#447005}