vaapi: support allocating buffers without GL

VDAs can be created without any GL context. In this case,
VaapiPictureFactory fails to return pictures since it cannot rely on the
GL implementation to decide an allocation method. Turn to Ozone in such
a case, since this is the most sensible candidate and we plan to use
this feature in unit tests mainly.

BUG=819542
TEST=Verified that vdatest was still passing on eve, hana and kevin

Change-Id: I08484d1619f32a6bf7613f3cb4663314a1f67c72
Reviewed-on: https://chromium-review.googlesource.com/961779Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544979}

media/gpu/vaapi/vaapi_picture_factory.cc

@@ -54,19 +54,24 @@ std::unique_ptr<VaapiPicture> VaapiPictureFactory::Create(
 
   // Select DRM(egl) / TFP(glx) at runtime with --use-gl=egl / --use-gl=desktop
   switch (GetVaapiImplementation(gl::GetGLImplementation())) {
-    case kVaapiImplementationDrm:
 #if defined(USE_OZONE)
+    // We can be called without GL initialized, which is valid if we use Ozone.
+    case kVaapiImplementationNone:
+      FALLTHROUGH;
+    case kVaapiImplementationDrm:
       picture.reset(new VaapiPictureNativePixmapOzone(
           vaapi_wrapper, make_context_current_cb, bind_image_cb,
           picture_buffer_id, size, texture_id, client_texture_id,
           texture_target));
+      break;
 #elif defined(USE_EGL)
+    case kVaapiImplementationDrm:
       picture.reset(new VaapiPictureNativePixmapEgl(
           vaapi_wrapper, make_context_current_cb, bind_image_cb,
           picture_buffer_id, size, texture_id, client_texture_id,
           texture_target));
-#endif
       break;
+#endif
 
 #if defined(USE_X11)
     case kVaapiImplementationX11: