fix skpdiff viewer bug when using relative paths

tools/skpdiff/skpdiff_util.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #if SK_BUILD_FOR_MAC || SK_BUILD_FOR_UNIX || SK_BUILD_FOR_ANDROID
 #   include <unistd.h>
 #   include <sys/time.h>
 #   include <dirent.h>
 #endif
 
 #if SK_BUILD_FOR_MAC || SK_BUILD_FOR_UNIX
 #   include <glob.h>
 #endif
 
+#if SK_BUILD_FOR_MAC
+#   include <sys/syslimits.h> // PATH_MAX is here for Macs
+#endif
+
 #if SK_BUILD_FOR_WIN32
 #   include <windows.h>
 #endif
 
+#include <stdlib.h>
 #include <time.h>
 #include "SkOSFile.h"
 #include "skpdiff_util.h"
 
 #if SK_SUPPORT_OPENCL
 const char* cl_error_to_string(cl_int err) {
     switch (err) {
         case CL_SUCCESS:                         return "CL_SUCCESS";
         case CL_DEVICE_NOT_FOUND:                return "CL_DEVICE_NOT_FOUND";
         case CL_DEVICE_NOT_AVAILABLE:            return "CL_DEVICE_NOT_AVAILABLE";
@@ skipping 142 matching lines @@
         paths++;
     }
 
     globfree(&globBuffer);
 
     return true;
 #else
     return false;
 #endif
 }
+
+SkString get_absolute_path(const SkString& path) {
+#if SK_BUILD_FOR_MAC || SK_BUILD_FOR_UNIX || SK_BUILD_FOR_ANDROID
+    SkString fullPath(PATH_MAX + 1);
+    if (realpath(path.c_str(), fullPath.writable_str()) == NULL) {
+        fullPath.reset();
+    }
+    return fullPath;
+#elif SK_BUILD_FOR_WIN32
+    SkString fullPath(MAX_PATH);
+    if (_fullpath(fullPath.writable_str(), path.c_str(), MAX_PATH) == NULL) {
+        fullPath.reset();
+    }
+    return fullPath;
+#else
+    return SkString();
+#endif
+}