Move shader precision modifier check onto GLSLCaps

BUG=skia:

Review URL: https://codereview.chromium.org/1414373002
diff --git a/src/gpu/gl/GrGLGLSL.cpp b/src/gpu/gl/GrGLGLSL.cpp
index ea65842..8a308f3 100755
--- a/src/gpu/gl/GrGLGLSL.cpp
+++ b/src/gpu/gl/GrGLGLSL.cpp
@@ -6,7 +6,9 @@
  */
 
 #include "GrGLGLSL.h"
+#include "GrGLContext.h"
 #include "GrGLShaderVar.h"
+#include "GrGLUtil.h"
 #include "SkString.h"
 
 bool GrGLGetGLSLGeneration(const GrGLInterface* gl, GrGLSLGeneration* generation) {