Add GYP target for sk_tool_utils.* component.

This declares a static library target in gyp/sk_tool_utils.gyp, so other
targets can depend directly on it instead of including the source file
in their source lists.

BUG=None
TEST=make all
R=mtklein@google.com

Author: tfarina@chromium.org

Review URL: https://codereview.chromium.org/348623006
diff --git a/tools/sk_tool_utils.h b/tools/sk_tool_utils.h
index 48fd716..2bd42b6 100644
--- a/tools/sk_tool_utils.h
+++ b/tools/sk_tool_utils.h
@@ -8,8 +8,10 @@
 #ifndef sk_tool_utils_DEFINED
 #define sk_tool_utils_DEFINED
 
-#include "SkCanvas.h"
-#include "SkBitmap.h"
+#include "SkImageInfo.h"
+
+class SkBitmap;
+class SkCanvas;
 
 namespace sk_tool_utils {
 
@@ -20,6 +22,7 @@
      *  the pixels are colorType + alphaType
      */
     void write_pixels(SkCanvas*, const SkBitmap&, int x, int y, SkColorType, SkAlphaType);
-}
 
-#endif
+}  // namespace sk_tool_utils
+
+#endif  // sk_tool_utils_DEFINED