Allow skimage expectations file to not exist.

tools/skimage_main.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm_expectations.h"
 #include "SkBitmap.h"
 #include "SkColorPriv.h"
@@ skipping 466 matching lines @@
     SkCommandLineFlags::Parse(argc, argv);
 
     if (FLAGS_readPath.count() < 1) {
         SkDebugf("Folder(s) or image(s) to decode are required.\n");
         return -1;
     }
 
 
     SkAutoGraphics ag;
 
-    if (!FLAGS_readExpectationsPath.isEmpty()) {
+    if (!FLAGS_readExpectationsPath.isEmpty() && sk_exists(FLAGS_readExpectationsPath[0])) {
         gJsonExpectations.reset(SkNEW_ARGS(skiagm::JsonExpectationsSource,
                                            (FLAGS_readExpectationsPath[0])));
     }
 
     SkString outDir;
     SkString* outDirPtr;
 
     if (FLAGS_writePath.count() == 1) {
         outDir.set(FLAGS_writePath[0]);
         append_path_separator_if_necessary(&outDir);
@@ skipping 46 matching lines @@
     }
 
     return failed ? -1 : 0;
 }
 
 #if !defined SK_BUILD_FOR_IOS
 int main(int argc, char * const argv[]) {
     return tool_main(argc, (char**) argv);
 }
 #endif