add toString to SkImage

src/image/SkImage.cpp

Index: src/image/SkImage.cpp
diff --git a/src/image/SkImage.cpp b/src/image/SkImage.cpp
index 1acff3e4ed5ff7bad77e7bc63643c5bda0f39974..8796b0328355f1c5049ee0151454197c74acbc57 100644
--- a/src/image/SkImage.cpp
+++ b/src/image/SkImage.cpp
@@ -10,6 +10,7 @@
 #include "SkImagePriv.h"
 #include "SkImage_Base.h"
 #include "SkReadPixelsRec.h"
+#include "SkString.h"
 #include "SkSurface.h"
 
 uint32_t SkImage::NextUniqueID() {
@@ -78,6 +79,12 @@ SkSurface* SkImage::newSurface(const SkImageInfo& info, const SkSurfaceProps* pr
     return as_IB(this)->onNewSurface(info, *props);
 }
 
+const char* SkImage::toString(SkString* str) const {
+    str->appendf("image: (id:%d (%d, %d) %s)", this->uniqueID(), this->width(), this->height(),
+                 this->isOpaque() ? "opaque" : "");
+    return str->c_str();
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 static bool raster_canvas_supports(const SkImageInfo& info) {