Move shader precision modifier check onto GLSLCaps

BUG=skia:

Review URL: https://codereview.chromium.org/1414373002
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index a66223d..2fe2382 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -908,6 +908,7 @@
 
 #include "effects/GrTextureStripAtlas.h"
 #include "GrInvariantOutput.h"
+#include "gl/GrGLContext.h"
 #include "gl/builders/GrGLProgramBuilder.h"
 #include "SkGr.h"