Add isSingleComponent bool to getConstantColorComponent

Initial step to allowing effects to use/output 1 or 4 color/coverage components. This cl doesn't change any current logic and all effects still assume they are working with 4 components.

BUG=skia:

Review URL: https://codereview.chromium.org/608253002
diff --git a/src/gpu/effects/GrConvexPolyEffect.h b/src/gpu/effects/GrConvexPolyEffect.h
index e474939..7f76f79 100644
--- a/src/gpu/effects/GrConvexPolyEffect.h
+++ b/src/gpu/effects/GrConvexPolyEffect.h
@@ -70,8 +70,6 @@
 
     typedef GrGLConvexPolyEffect GLProcessor;
 
-    virtual void getConstantColorComponents(GrColor* color, uint32_t* validFlags) const SK_OVERRIDE;
-
     virtual const GrBackendFragmentProcessorFactory& getFactory() const SK_OVERRIDE;
 
 private:
@@ -79,6 +77,8 @@
 
     virtual bool onIsEqual(const GrProcessor& other) const SK_OVERRIDE;
 
+    virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE;
+
     GrPrimitiveEdgeType    fEdgeType;
     int                    fEdgeCount;
     SkScalar               fEdges[3 * kMaxEdges];