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 8efb98671387724061ebcee1ee78e62808268c23..55122d4ab89cc2342a3594dd894141e1bf2a87c7 100644
--- a/gm/gm_expectations.h
+++ b/gm/gm_expectations.h
@@ -32,16 +32,6 @@ 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 CreateJsonTree(Json::Value expectedResults,
                                Json::Value actualResultsFailed,
                                Json::Value actualResultsFailureIgnored,