Return an Error from Framebuffer::syncState.

This pipes errors up from the Impl to the top level. There are
still a few places were error swallowing is needed, because the
Framebuffer API doesn't support returning an error.

Bug: angleproject:2372
Change-Id: Idc06bda1817fd28075940f69874d8b6ba69194f9
Reviewed-on: https://chromium-review.googlesource.com/954290
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/FramebufferNULL.cpp b/src/libANGLE/renderer/null/FramebufferNULL.cpp
index 71d3cdc..30dba62 100644
--- a/src/libANGLE/renderer/null/FramebufferNULL.cpp
+++ b/src/libANGLE/renderer/null/FramebufferNULL.cpp
@@ -185,9 +185,10 @@
     return true;
 }
 
-void FramebufferNULL::syncState(const gl::Context *context,
-                                const gl::Framebuffer::DirtyBits &dirtyBits)
+gl::Error FramebufferNULL::syncState(const gl::Context *context,
+                                     const gl::Framebuffer::DirtyBits &dirtyBits)
 {
+    return gl::NoError();
 }
 
 gl::Error FramebufferNULL::getSamplePosition(size_t index, GLfloat *xy) const