Add some backend flexibility for shader declarations in ProgramBuilding.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1682703003

Review URL: https://codereview.chromium.org/1682703003
diff --git a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp
index f97b854..7ec18a2 100644
--- a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp
+++ b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.cpp
@@ -216,6 +216,7 @@
         fOutputs.push_back().set(kVec4f_GrSLType,
                                  GrGLSLShaderVar::kOut_TypeModifier,
                                  DeclaredColorOutputName());
+        fProgramBuilder->finalizeFragmentOutputColor(fOutputs.back());
     }
 }
 
@@ -234,6 +235,7 @@
     if (caps.mustDeclareFragmentShaderOutput()) {
         fOutputs.push_back().set(kVec4f_GrSLType, GrGLSLShaderVar::kOut_TypeModifier,
                                  DeclaredSecondaryColorOutputName());
+        fProgramBuilder->finalizeFragmentSecondaryColor(fOutputs.back());
     }
 }