make tests portable by using 565 compatible colors

TBR=reed@google.com

Review URL: https://codereview.chromium.org/1262703002
diff --git a/gm/shallowgradient.cpp b/gm/shallowgradient.cpp
index 415ed41..276ec14 100644
--- a/gm/shallowgradient.cpp
+++ b/gm/shallowgradient.cpp
@@ -52,7 +52,8 @@
     }
 
     void onDraw(SkCanvas* canvas) override {
-        const SkColor colors[] = { 0xFF555555, 0xFF444444 };
+        const SkColor colors[] = { sk_tool_utils::color_to_565(0xFF555555), 
+                sk_tool_utils::color_to_565(0xFF444444) };
         const int colorCount = SK_ARRAY_COUNT(colors);
 
         SkRect r = { 0, 0, this->width(), this->height() };