Add onGetBlendInfo to GrXferProcessor

Adds an onGetBlendInfo method for GrXferProcessor subclasses to
override instead of overriding getBlendInfo directly. This gives the
base class a chance to initialize the struct with default values
before passing it on. As the BlendInfo struct grows, this will keep
things simple and less error prone.

BUG=skia:
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1049143002
diff --git a/src/gpu/effects/GrPorterDuffXferProcessor.cpp b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
index 2735ac3..77b7fca 100644
--- a/src/gpu/effects/GrPorterDuffXferProcessor.cpp
+++ b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
@@ -90,7 +90,16 @@
                                                GrColor* overrideColor,
                                                const GrDrawTargetCaps& caps) override;
 
-    void getBlendInfo(GrXferProcessor::BlendInfo* blendInfo) const override {
+    GrBlendCoeff getSrcBlend() const { return fSrcBlend; }
+    GrBlendCoeff getDstBlend() const { return fDstBlend; }
+
+private:
+    PorterDuffXferProcessor(GrBlendCoeff srcBlend, GrBlendCoeff dstBlend, GrColor constant,
+                            const GrDeviceCoordTexture* dstCopy, bool willReadDstColor);
+
+    void onGetGLProcessorKey(const GrGLCaps& caps, GrProcessorKeyBuilder* b) const override;
+
+    void onGetBlendInfo(GrXferProcessor::BlendInfo* blendInfo) const override {
         if (!this->willReadDstColor()) {
             blendInfo->fSrcBlend = fSrcBlend;
             blendInfo->fDstBlend = fDstBlend;
@@ -101,15 +110,6 @@
         blendInfo->fBlendConstant = fBlendConstant;
     }
 
-    GrBlendCoeff getSrcBlend() const { return fSrcBlend; }
-    GrBlendCoeff getDstBlend() const { return fDstBlend; }
-
-private:
-    PorterDuffXferProcessor(GrBlendCoeff srcBlend, GrBlendCoeff dstBlend, GrColor constant,
-                            const GrDeviceCoordTexture* dstCopy, bool willReadDstColor);
-
-    void onGetGLProcessorKey(const GrGLCaps& caps, GrProcessorKeyBuilder* b) const override;
-
     bool onIsEqual(const GrXferProcessor& xpBase) const override {
         const PorterDuffXferProcessor& xp = xpBase.cast<PorterDuffXferProcessor>();
         if (fSrcBlend != xp.fSrcBlend ||