Move glsl onto EmitArgs struct for emitCode

BUG=skia:

Review URL: https://codereview.chromium.org/1453623003
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index b60ee97..0bb8969 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -608,6 +608,7 @@
     t.append(".x");
     this->emitColor(args.fBuilder,
                     args.fFragBuilder,
+                    args.fGLSLCaps,
                     ge, t.c_str(),
                     args.fOutputColor,
                     args.fInputColor,