Move shader precision modifier check onto GLSLCaps

BUG=skia:

Review URL: https://codereview.chromium.org/1414373002
diff --git a/src/gpu/gl/GrGLCaps.h b/src/gpu/gl/GrGLCaps.h
index 105549f..34dde70 100644
--- a/src/gpu/gl/GrGLCaps.h
+++ b/src/gpu/gl/GrGLCaps.h
@@ -266,7 +266,7 @@
     bool rgba8888PixelsOpsAreSlow() const { return fRGBA8888PixelsOpsAreSlow; }
     bool partialFBOReadIsSlow() const { return fPartialFBOReadIsSlow; }
 
-    GrGLSLCaps* glslCaps() const { return reinterpret_cast<GrGLSLCaps*>(fShaderCaps.get()); }
+    const GrGLSLCaps* glslCaps() const { return reinterpret_cast<GrGLSLCaps*>(fShaderCaps.get()); }
 
 private:
     void init(const GrContextOptions&, const GrGLContextInfo&, const GrGLInterface*);