Add flag to skimage for decoding into unpremul.

Review URL: https://codereview.chromium.org/666073006
diff --git a/tools/skimage_main.cpp b/tools/skimage_main.cpp
index a2d353c..7a1954a 100644
--- a/tools/skimage_main.cpp
+++ b/tools/skimage_main.cpp
@@ -34,6 +34,7 @@
 DEFINE_bool(writeChecksumBasedFilenames, false,  "When writing out actual images, use checksum-"
             "based filenames, as rebaseline.py will use when downloading them from Google Storage");
 DEFINE_string2(writePath, w, "",  "Write rendered images into this directory.");
+DEFINE_bool(unpremul, false, "Require unpremultiplied colors.");
 
 struct Format {
     SkImageEncoder::Type    fType;
@@ -499,6 +500,7 @@
 
     codec->setSkipWritingZeroes(FLAGS_skip);
     codec->setSampleSize(FLAGS_sampleSize);
+    codec->setRequireUnpremultipliedColors(FLAGS_unpremul);
     stream.rewind();
 
     // Create a string representing just the filename itself, for use in json expectations.