GM: replace boilerplate with macros

I have verified locally that nothing draws differently.

Motivation:
*   SK_SIMPLE_GM makes it easier to write a GM.
*   Reducing 1100 lines of code makes maintenance easier.
*   Simple GMs are easy to convert to Fiddles.

Review URL: https://codereview.chromium.org/1333553002
diff --git a/gm/gradient_matrix.cpp b/gm/gradient_matrix.cpp
index 7722759..8f665a1 100644
--- a/gm/gradient_matrix.cpp
+++ b/gm/gradient_matrix.cpp
@@ -111,25 +111,8 @@
     canvas->restore();
 }
 
-namespace skiagm {
-
-class GradientMatrixGM : public GM {
-public:
-    GradientMatrixGM() {
-        this->setBGColor(sk_tool_utils::color_to_565(0xFFDDDDDD));
-    }
-
-protected:
-
-    SkString onShortName() override {
-        return SkString("gradient_matrix");
-    }
-
-    SkISize onISize() override {
-        return SkISize::Make(800, 800);
-    }
-
-    void onDraw(SkCanvas* canvas) override {
+DEF_SIMPLE_GM_BG(gradient_matrix, canvas, 800, 800,
+                 sk_tool_utils::color_to_565(0xFFDDDDDD)) {
         draw_gradients(canvas, &make_linear_gradient,
                       linearPts, SK_ARRAY_COUNT(linearPts));
 
@@ -137,11 +120,4 @@
 
         draw_gradients(canvas, &make_radial_gradient,
                       radialPts, SK_ARRAY_COUNT(radialPts));
-    }
-
-private:
-    typedef GM INHERITED;
-};
-
-DEF_GM( return new GradientMatrixGM; )
 }