added support for glMinSampleShading
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1812223002

Review URL: https://codereview.chromium.org/1812223002
diff --git a/src/gpu/GrPrimitiveProcessor.h b/src/gpu/GrPrimitiveProcessor.h
index e8bb449..3300ec7 100644
--- a/src/gpu/GrPrimitiveProcessor.h
+++ b/src/gpu/GrPrimitiveProcessor.h
@@ -217,6 +217,10 @@
      */
     virtual const char* getDestColorOverride() const { return nullptr; }
     
+    virtual float getSampleShading() const {
+        return 0.0;
+    }
+
 protected:
     GrPrimitiveProcessor() : fVertexStride(0) {}