Fixes for Angle to build with Windows GN.

Change-Id: Id38c4b11dba31b2779efa7beade481cd50f3d935
Reviewed-on: https://chromium-review.googlesource.com/227692
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>

BUILD.gn

@@ -226,6 +226,10 @@ if (is_win) {
       "//build/config/compiler:no_chromium_code",
     ]
 
+    if (angle_enable_d3d11) {
+      defines += [ "ANGLE_ENABLE_D3D11" ]
+    }
+
     if (is_debug) {
       defines += [
         "ANGLE_ENABLE_PERF",