Add path utils, plus a test for it.

gm/gm_expectations.h

Index: gm/gm_expectations.h
diff --git a/gm/gm_expectations.h b/gm/gm_expectations.h
index a8b29cd6bd5ec331ec0a9393e8ebd9e1b1e9f628..60430850e03a914ad03431e501e32b36780d6d2a 100644
--- a/gm/gm_expectations.h
+++ b/gm/gm_expectations.h
@@ -41,16 +41,6 @@ namespace skiagm {
 
     void gm_fprintf(FILE *stream, const char format[], ...);
 
-    /**
-     * Assembles rootPath and relativePath into a single path, like this:
-     * rootPath/relativePath
-     *
-     * Uses SkPATH_SEPARATOR, to work on all platforms.
-     *
-     * TODO(epoger): This should probably move into SkOSFile.h
-     */
-    SkString SkPathJoin(const char *rootPath, const char *relativePath);
-
     Json::Value ActualResultAsJsonValue(const SkHashDigest& result);
 
     Json::Value CreateJsonTree(Json::Value expectedResults,