Add cap and builder feature for multisample interpolation

src/gpu/glsl/GrGLSLCaps.h

Index: src/gpu/glsl/GrGLSLCaps.h
diff --git a/src/gpu/glsl/GrGLSLCaps.h b/src/gpu/glsl/GrGLSLCaps.h
index 914b0dbb4ce7fab8a2a578a3f7726f2ccb80151a..1c799454a7c31f3d1aefd7daf05fb59398d50acc 100755
--- a/src/gpu/glsl/GrGLSLCaps.h
+++ b/src/gpu/glsl/GrGLSLCaps.h
@@ -58,6 +58,8 @@ public:
 
     bool noperspectiveInterpolationSupport() const { return fNoPerspectiveInterpolationSupport; }
 
+    bool multisampleInterpolationSupport() const { return fMultisampleInterpolationSupport; }
+
     bool sampleVariablesSupport() const { return fSampleVariablesSupport; }
 
     bool sampleMaskOverrideCoverageSupport() const { return fSampleMaskOverrideCoverageSupport; }
@@ -118,6 +120,11 @@ public:
         return fNoPerspectiveInterpolationExtensionString;
     }
 
+    const char* multisampleInterpolationExtensionString() const {
+        SkASSERT(this->multisampleInterpolationSupport());
+        return fMultisampleInterpolationExtensionString;
+    }
+
     const char* sampleVariablesExtensionString() const {
         SkASSERT(this->sampleVariablesSupport());
         return fSampleVariablesExtensionString;
@@ -157,6 +164,7 @@ private:
     bool fCanUseAnyFunctionInShader : 1;
     bool fFlatInterpolationSupport : 1;
     bool fNoPerspectiveInterpolationSupport : 1;
+    bool fMultisampleInterpolationSupport : 1;
     bool fSampleVariablesSupport : 1;
     bool fSampleMaskOverrideCoverageSupport : 1;
 
@@ -171,6 +179,7 @@ private:
     const char* fSecondaryOutputExtensionString;
     const char* fExternalTextureExtensionString;
     const char* fNoPerspectiveInterpolationExtensionString;
+    const char* fMultisampleInterpolationExtensionString;
     const char* fSampleVariablesExtensionString;
 
     const char* fFBFetchColorName;