Minor code cleanup (left over from prior CL)

This was requested in https://codereview.chromium.org/1309543012/ (Add new GM to directly exercise perspective bitmap drawing)

Review URL: https://codereview.chromium.org/1320673011
diff --git a/gm/xfermodeimagefilter.cpp b/gm/xfermodeimagefilter.cpp
index 80994a6..e09c0ab 100644
--- a/gm/xfermodeimagefilter.cpp
+++ b/gm/xfermodeimagefilter.cpp
@@ -29,19 +29,6 @@
         return SkString("xfermodeimagefilter");
     }
 
-    void make_bitmap() {
-        fBitmap.allocN32Pixels(80, 80);
-        SkCanvas canvas(fBitmap);
-        canvas.clear(0x00000000);
-        SkPaint paint;
-        paint.setAntiAlias(true);
-        sk_tool_utils::set_portable_typeface(&paint);
-        paint.setColor(0xD000D000);
-        paint.setTextSize(SkIntToScalar(96));
-        const char* str = "e";
-        canvas.drawText(str, strlen(str), SkIntToScalar(15), SkIntToScalar(65), paint);
-    }
-
     SkISize onISize() override {
         return SkISize::Make(WIDTH, HEIGHT);
     }
@@ -66,13 +53,13 @@
     }
 
     void onOnceBeforeDraw() override {
-        make_bitmap();
+        fBitmap = sk_tool_utils::create_string_bitmap(80, 80, 0xD000D000, 15, 65, 96, "e");
 
-        fCheckerboard.allocN32Pixels(80, 80);
-        SkCanvas checkerboardCanvas(fCheckerboard);
-        sk_tool_utils::draw_checkerboard(&checkerboardCanvas,
-                sk_tool_utils::color_to_565(0xFFA0A0A0),
-                sk_tool_utils::color_to_565(0xFF404040), 8);
+        fCheckerboard = sk_tool_utils::create_checkerboard_bitmap(
+                                                        80, 80,
+                                                        sk_tool_utils::color_to_565(0xFFA0A0A0),
+                                                        sk_tool_utils::color_to_565(0xFF404040),
+                                                        8);
     }
 
     void onDraw(SkCanvas* canvas) override {