Return an Error from ContextImpl::syncState.

This will allow the Vulkan back-end to call Vulkan commands that return
VkResult within syncState.

Bug: angleproject:2717
Change-Id: I05e1379e71acaac56bcc9493c8561e82da000934
Reviewed-on: https://chromium-review.googlesource.com/1120150
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
diff --git a/src/libANGLE/renderer/null/ContextNULL.cpp b/src/libANGLE/renderer/null/ContextNULL.cpp
index d7e5e6c..b288c23 100644
--- a/src/libANGLE/renderer/null/ContextNULL.cpp
+++ b/src/libANGLE/renderer/null/ContextNULL.cpp
@@ -295,8 +295,9 @@
 {
 }
 
-void ContextNULL::syncState(const gl::Context *context, const gl::State::DirtyBits &dirtyBits)
+gl::Error ContextNULL::syncState(const gl::Context *context, const gl::State::DirtyBits &dirtyBits)
 {
+    return gl::NoError();
 }
 
 GLint ContextNULL::getGPUDisjoint()