Add path utils, plus a test for it.

gm/factory.cpp

Index: gm/factory.cpp
diff --git a/gm/factory.cpp b/gm/factory.cpp
index 4538cda8945d007fdbbbeb859af86a075f16c9c7..a7356a96e30a61288c5be5aaa883a201fbe21927 100644
--- a/gm/factory.cpp
+++ b/gm/factory.cpp
@@ -11,6 +11,7 @@
 #include "SkData.h"
 #include "SkImageDecoder.h"
 #include "SkLruImageCache.h"
+#include "SkOSFile.h"
 #include "SkStream.h"
 
 namespace skiagm {
@@ -24,13 +25,9 @@ public:
 
 protected:
     virtual void onOnceBeforeDraw() SK_OVERRIDE {
-        SkString filename(INHERITED::gResourcePath);
-        if (!filename.endsWith("/") && !filename.endsWith("\\")) {
-            filename.append("/");
-        }
-
         // Copyright-free file from http://openclipart.org/detail/29213/paper-plane-by-ddoo
-        filename.append("plane.png");
+        SkString filename = SkOSPath::SkPathJoin(INHERITED::gResourcePath.c_str(),
+                                                 "plane.png");
 
         SkAutoTUnref<SkStream> stream(SkStream::NewFromFile(filename.c_str()));
         if (NULL != stream.get()) {