Remove uses of SkImageDecoder from skdiff

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1788643003

Review URL: https://codereview.chromium.org/1788643003
diff --git a/tools/skdiff_utils.h b/tools/skdiff_utils.h
index 8b94117..ae7d51a 100644
--- a/tools/skdiff_utils.h
+++ b/tools/skdiff_utils.h
@@ -9,7 +9,6 @@
 #define skdiff_utils_DEFINED
 
 #include "skdiff.h"
-#include "SkImageDecoder.h"
 
 class SkBitmap;
 class SkData;
@@ -26,7 +25,7 @@
 SkData* read_file(const char* file_path);
 
 /** Decodes the fileBits into the resource.fBitmap. Returns false on failure. */
-bool get_bitmap(SkData* fileBits, DiffResource& resource, SkImageDecoder::Mode mode);
+bool get_bitmap(SkData* fileBits, DiffResource& resource, bool sizeOnly);
 
 /** Writes the bitmap as a PNG to the path specified. */
 bool write_bitmap(const SkString& path, const SkBitmap& bitmap);