Add standard way to indicate GM is GPU-only.

gm/dcshader.cpp

Index: gm/dcshader.cpp
diff --git a/gm/dcshader.cpp b/gm/dcshader.cpp
index 1f5969bd3c6eefee456ea3d09bd565e076d9b25e..4b8f5eddc7492cf961b5d7e913fe20ab2145074a 100644
--- a/gm/dcshader.cpp
+++ b/gm/dcshader.cpp
@@ -248,6 +248,12 @@ protected:
     }
 
     void onDraw(SkCanvas* canvas) SK_OVERRIDE {
+        // This GM exists to test a specific feature of the GPU backend. It does not work with the
+        // sw rasterizer, tile modes, etc.
+        if (NULL == canvas->getGrContext()) {
+            this->drawGpuOnlyMessage(canvas);
+            return;
+        }
         SkPaint paint;
         SkTArray<SkMatrix> devMats;
         devMats.push_back().reset();