Add path utils, plus a test for it.

gm/gm_expectations.cpp

Index: gm/gm_expectations.cpp
diff --git a/gm/gm_expectations.cpp b/gm/gm_expectations.cpp
index 8138af78907e3fd34286d69f9f524b0d0fa1cf16..46cfea12366f8c5e7ab5d050ae98f398acc6a18f 100644
--- a/gm/gm_expectations.cpp
+++ b/gm/gm_expectations.cpp
@@ -37,15 +37,6 @@ namespace skiagm {
         va_end(args);
     }
 
-    SkString SkPathJoin(const char *rootPath, const char *relativePath) {
-        SkString result(rootPath);
-        if (!result.endsWith(SkPATH_SEPARATOR)) {
-            result.appendUnichar(SkPATH_SEPARATOR);
-        }
-        result.append(relativePath);
-        return result;
-    }
-
     Json::Value CreateJsonTree(Json::Value expectedResults,
                                Json::Value actualResultsFailed,
                                Json::Value actualResultsFailureIgnored,
@@ -194,7 +185,7 @@ namespace skiagm {
     // IndividualImageExpectationsSource class...
 
     Expectations IndividualImageExpectationsSource::get(const char *testName) {
-        SkString path = SkPathJoin(fRootDir.c_str(), testName);
+        SkString path = SkOSPath::SkPathJoin(fRootDir.c_str(), testName);
         SkBitmap referenceBitmap;
         bool decodedReferenceBitmap =
             SkImageDecoder::DecodeFile(path.c_str(), &referenceBitmap,