Add toString methods to SkImageFilter-derived classes

This isn't definitive but at least makes something show up in the debugger.

Review URL: https://codereview.chromium.org/789163006
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index 3b73eae..d50afb7 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -704,7 +704,7 @@
     str->appendf("%d colors: ", fColorCount);
 
     for (int i = 0; i < fColorCount; ++i) {
-        str->appendHex(fOrigColors[i]);
+        str->appendHex(fOrigColors[i], 8);
         if (i < fColorCount-1) {
             str->append(", ");
         }