Check for xfer barriers in GrBatch, auto-issue barriers in GrGpu

src/gpu/gl/GrGLGpu.cpp

Index: src/gpu/gl/GrGLGpu.cpp
diff --git a/src/gpu/gl/GrGLGpu.cpp b/src/gpu/gl/GrGLGpu.cpp
index ec95557655b55739aa9d4f7cc0e2c964cecfa996..0945ed565131e78c753ae9095a4269d6414eb48f 100644
--- a/src/gpu/gl/GrGLGpu.cpp
+++ b/src/gpu/gl/GrGLGpu.cpp
@@ -3062,6 +3062,7 @@ bool GrGLGpu::copySurfaceAsBlitFramebuffer(GrSurface* dst,
 }
 
 void GrGLGpu::xferBarrier(GrRenderTarget* rt, GrXferBarrierType type) {
+    SkASSERT(type);
     switch (type) {
         case kTexture_GrXferBarrierType: {
             GrGLRenderTarget* glrt = static_cast<GrGLRenderTarget*>(rt);
@@ -3080,6 +3081,7 @@ void GrGLGpu::xferBarrier(GrRenderTarget* rt, GrXferBarrierType type) {
                      this->caps()->blendEquationSupport());
             GL_CALL(BlendBarrier());
             return;
+        default: break; // placate compiler warnings that kNone not handled
     }
 }