Allow skimage expectations file to not exist.

If the file is not there, assume that all images are mismatches.
This allows running the test before the initial rebaseline.

R=epoger@google.com

Review URL: https://codereview.chromium.org/17448011

git-svn-id: http://skia.googlecode.com/svn/trunk@9690 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/skimage_main.cpp b/tools/skimage_main.cpp
index 107d3bf..a6a2736 100644
--- a/tools/skimage_main.cpp
+++ b/tools/skimage_main.cpp
@@ -484,7 +484,7 @@
 
     SkAutoGraphics ag;
 
-    if (!FLAGS_readExpectationsPath.isEmpty()) {
+    if (!FLAGS_readExpectationsPath.isEmpty() && sk_exists(FLAGS_readExpectationsPath[0])) {
         gJsonExpectations.reset(SkNEW_ARGS(skiagm::JsonExpectationsSource,
                                            (FLAGS_readExpectationsPath[0])));
     }