Eliminate one copy of replace_char() function.

tools/skpdiff/SkDiffContext.cpp

Index: tools/skpdiff/SkDiffContext.cpp
diff --git a/tools/skpdiff/SkDiffContext.cpp b/tools/skpdiff/SkDiffContext.cpp
index 32688f5013ce75b593ec843a5b8d72b3f59c68b7..18ed043953bc6ac6dd223f216108d1ec367fafb9 100644
--- a/tools/skpdiff/SkDiffContext.cpp
+++ b/tools/skpdiff/SkDiffContext.cpp
@@ -6,19 +6,15 @@
  */
 
 #include "SkBitmap.h"
+#include "SkDiffContext.h"
 #include "SkImageDecoder.h"
+#include "SkImageDiffer.h"
 #include "SkOSFile.h"
 #include "SkRunnable.h"
 #include "SkSize.h"
 #include "SkStream.h"
 #include "SkTDict.h"
 #include "SkTaskGroup.h"
-
-// from the tools directory for replace_char(...)
-#include "picture_utils.h"
-
-#include "SkDiffContext.h"
-#include "SkImageDiffer.h"
 #include "skpdiff_util.h"
 
 SkDiffContext::SkDiffContext() {
@@ -91,7 +87,7 @@ static SkString get_combined_name(const SkString& a, const SkString& b) {
     SkString result = a;
     result.append("-vs-");
     result.append(b);
-    sk_tools::replace_char(&result, '.', '_');
+    result.replace('.', '_');
     return result;
 }