Add gl_SampleMask functionality to fragment builders

src/gpu/glsl/GrGLSLCaps.cpp

Index: src/gpu/glsl/GrGLSLCaps.cpp
diff --git a/src/gpu/glsl/GrGLSLCaps.cpp b/src/gpu/glsl/GrGLSLCaps.cpp
index ba99be57e26ddb44b30a3728ee5ef1dda417c6ca..0aa3e4ffeac5f3faed509a52d0464ff5e72f95d5 100755
--- a/src/gpu/glsl/GrGLSLCaps.cpp
+++ b/src/gpu/glsl/GrGLSLCaps.cpp
@@ -25,12 +25,15 @@ GrGLSLCaps::GrGLSLCaps(const GrContextOptions& options) {
     fMustForceNegatedAtanParamToFloat = false;
     fFlatInterpolationSupport = false;
     fNoPerspectiveInterpolationSupport = false;
+    fSampleVariablesSupport = false;
+    fSampleMaskOverrideCoverageSupport = false;
     fVersionDeclString = nullptr;
     fShaderDerivativeExtensionString = nullptr;
     fFragCoordConventionsExtensionString = nullptr;
     fSecondaryOutputExtensionString = nullptr;
     fExternalTextureExtensionString = nullptr;
     fNoPerspectiveInterpolationExtensionString = nullptr;
+    fSampleVariablesExtensionString = nullptr;
     fFBFetchColorName = nullptr;
     fFBFetchExtensionString = nullptr;
     fAdvBlendEqInteraction = kNotSupported_AdvBlendEqInteraction;
@@ -64,6 +67,9 @@ SkString GrGLSLCaps::dump() const {
     r.appendf("Flat interpolation support: %s\n", (fFlatInterpolationSupport ?  "YES" : "NO"));
     r.appendf("No perspective interpolation support: %s\n", (fNoPerspectiveInterpolationSupport ?
                                                              "YES" : "NO"));
+    r.appendf("Sample variables support: %s\n", (fSampleVariablesSupport ? "YES" : "NO"));
+    r.appendf("Sample mask override coverage support: %s\n", (fSampleMaskOverrideCoverageSupport ?
+                                                              "YES" : "NO"));
     r.appendf("Advanced blend equation interaction: %s\n",
               kAdvBlendEqInteractionStr[fAdvBlendEqInteraction]);
     return r;