Lock around all EGL and GL calls with a global mutex.

BUG=angleproject:2464

Change-Id: I0231cc84777272f9cf26298c6a137f1ad3fd51d6
Reviewed-on: https://chromium-review.googlesource.com/1183441
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index d614fc7..ca4445b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -76,6 +76,10 @@
   } else {
     defines += [ "ANGLE_IS_32_BIT_CPU" ]
   }
+
+  if (angle_force_thread_safety) {
+    defines += [ "ANGLE_FORCE_THREAD_SAFETY=1" ]
+  }
 }
 
 config("extra_warnings") {