Fixed GM:lightingshader2 size

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2171853002

Review-Url: https://codereview.chromium.org/2171853002
diff --git a/gm/lightingshader2.cpp b/gm/lightingshader2.cpp
index c2f4d6c..9283449 100644
--- a/gm/lightingshader2.cpp
+++ b/gm/lightingshader2.cpp
@@ -38,7 +38,7 @@
     }
 
     SkISize onISize() override {
-        return SkISize::Make(kGMSize, kGMSize);
+        return SkISize::Make(600, 740);
     }
 
     void onOnceBeforeDraw() override {
@@ -214,7 +214,6 @@
 
 private:
     static const int kTexSize = 96;
-    static const int kGMSize  = 512;
 
     sk_sp<SkShader> fOpaqueDiffuse;
     sk_sp<SkShader> fTranslucentDiffuse;