Added missing entries for SwiftShader on ChromeOS

On ChromeOS, for both gbm and wayland platforms, SwiftShader had been
added to the list of allowed GL implementations without being added to
their corresponding GetGLOzone functions. This adds the missing entries.

Also, on gbm, the GetGLOzone function had a thread checker, which no
other platform has. Since it was making the SwiftShader implementation
fail, I removed it. It may have been a mistake in the conversion
process and had been added here:
https://codereview.chromium.org/2327813002/diff/20001/ui/ozone/platform/drm/gpu/gbm_surface_factory.cc

Bug: chromium:848035
Change-Id: I8c88dcd84974668b935c96e669a4b2c642a3ba21
Reviewed-on: https://chromium-review.googlesource.com/1111332Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569679}

ui/ozone/platform/drm/gpu/gbm_surface_factory.cc

@@ -114,9 +114,9 @@ GbmSurfaceFactory::GetAllowedGLImplementations() {
 }
 
 GLOzone* GbmSurfaceFactory::GetGLOzone(gl::GLImplementation implementation) {
-  DCHECK(thread_checker_.CalledOnValidThread());
   switch (implementation) {
     case gl::kGLImplementationEGLGLES2:
+    case gl::kGLImplementationSwiftShaderGL:
       return egl_implementation_.get();
     case gl::kGLImplementationOSMesaGL:
       return osmesa_implementation_.get();

ui/ozone/platform/wayland/wayland_surface_factory.cc

@@ -218,6 +218,7 @@ GLOzone* WaylandSurfaceFactory::GetGLOzone(
     gl::GLImplementation implementation) {
   switch (implementation) {
     case gl::kGLImplementationEGLGLES2:
+    case gl::kGLImplementationSwiftShaderGL:
       return egl_implementation_.get();
     case gl::kGLImplementationOSMesaGL:
       return osmesa_implementation_.get();