Add some backend flexibility for shader declarations in ProgramBuilding.

src/gpu/glsl/GrGLSLProgramBuilder.h

Index: src/gpu/glsl/GrGLSLProgramBuilder.h
diff --git a/src/gpu/glsl/GrGLSLProgramBuilder.h b/src/gpu/glsl/GrGLSLProgramBuilder.h
index 2249c3c712ea631ebe4880785e85bb050589049f..a2775956121b1625c3197a86d90c4e052d8fe170 100644
--- a/src/gpu/glsl/GrGLSLProgramBuilder.h
+++ b/src/gpu/glsl/GrGLSLProgramBuilder.h
@@ -69,6 +69,11 @@ public:
     virtual const GrGLSLUniformHandler* uniformHandler() const = 0;
     virtual GrGLSLVaryingHandler* varyingHandler() = 0;
 
+    // Used for backend customization of the output color and secondary color variables from the
+    // fragment processor. Only used if the outputs are explicitly declared in the shaders
+    virtual void finalizeFragmentOutputColor(GrGLSLShaderVar& outputColor) {}
+    virtual void finalizeFragmentSecondaryColor(GrGLSLShaderVar& outputColor) {}
+
     // number of each input/output type in a single allocation block, used by many builders
     static const int kVarsPerBlock;