Multithreading in D3D11 minimum viable product

Bug: b/168046573
Change-Id: I676a148333cbf5e9ca508768503e62cb14d8eeb0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2629618
Commit-Queue: Doug Horn <doughorn@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Trevor David Black <vantablack@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 8044d97..2e4feab 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -615,6 +615,7 @@
 
   if (angle_enable_d3d11) {
     configs += [ "src/libANGLE/renderer/d3d:angle_d3d11_backend_config" ]
+    defines += [ "ANGLE_FORCE_CONTEXT_CHECK_EVERY_CALL=1" ]
   }
 
   if (angle_enable_d3d9) {