Fix iOS build by centralizing --writePath.

dm/DMWriteTask.cpp

 #include "DMWriteTask.h"
 
 #include "DMUtil.h"
 #include "SkColorPriv.h"
 #include "SkCommonFlags.h"
 #include "SkImageEncoder.h"
 #include "SkMallocPixelRef.h"
 #include "SkStream.h"
 #include "SkString.h"
 
-DEFINE_string2(writePath, w, "", "If set, write GMs here as .pngs.");
 DEFINE_bool(writePngOnly, false, "If true, don't encode raw bitmap after .png data.  "
                                  "This means -r won't work, but skdiff will still work fine.");
 
 namespace DM {
 
 // Splits off the last N suffixes of name (splitting on _) and appends them to out.
 // Returns the total number of characters consumed.
 static int split_suffixes(int N, const char* name, SkTArray<SkString>* out) {
     SkTArray<SkString> split;
     SkStrSplit(name, "_", &split);
@@ skipping 169 matching lines @@
     const SkString path = path_to_expected_image(fRoot, task);
     SkBitmap expected;
     if (!PngAndRaw::Decode(path.c_str(), bitmap.info(), &expected)) {
         return false;
     }
 
     return BitmapsEqual(expected, bitmap);
 }
 
 }  // namespace DM