Add option to dump images from nanobench.

src/utils/SkOSFile.cpp

Index: src/utils/SkOSFile.cpp
diff --git a/src/utils/SkOSFile.cpp b/src/utils/SkOSFile.cpp
index 8baf08ffdd394068e0dcc065964dcf54b45ff62a..04a4fe910e9ae2da0f9ed1fd604a8cb638891ed7 100644
--- a/src/utils/SkOSFile.cpp
+++ b/src/utils/SkOSFile.cpp
@@ -8,7 +8,7 @@
 
 SkString SkOSPath::Join(const char *rootPath, const char *relativePath) {
     SkString result(rootPath);
-    if (!result.endsWith(SkPATH_SEPARATOR)) {
+    if (!result.endsWith(SkPATH_SEPARATOR) && !result.isEmpty()) {
         result.appendUnichar(SkPATH_SEPARATOR);
     }
     result.append(relativePath);
@@ -28,6 +28,21 @@ SkString SkOSPath::Basename(const char* fullPath) {
     return SkString(filename);
 }
 
+SkString SkOSPath::Dirname(const char* fullPath) {
+    if (!fullPath) {
+        return SkString();
+    }
+    const char* end = strrchr(fullPath, SkPATH_SEPARATOR);
+    if (NULL == end) {
+        return SkString();
+    }
+    if (end == fullPath) {
+        SkASSERT(fullPath[0] == SkPATH_SEPARATOR);
+        ++end;
+    }
+    return SkString(fullPath, end - fullPath);
+}
+
 #ifdef SK_BUILD_FOR_WIN
 
 static uint16_t* concat_to_16(const char src[], const char suffix[])