Fix bug in setting directories for tests.

make_canonical_dir_path only worked if the provided directory
did not end with a slash. Remove this function, and call
SkPathJoin instead. Update the documentation to acknowledge
that this is an acceptable use of SkPathJoin, and update its
test.

R=epoger@google.com

Review URL: https://codereview.chromium.org/16098011

git-svn-id: http://skia.googlecode.com/svn/trunk@9458 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/StreamTest.cpp b/tests/StreamTest.cpp
index 8f38f46..00c079a 100644
--- a/tests/StreamTest.cpp
+++ b/tests/StreamTest.cpp
@@ -35,8 +35,7 @@
 }
 
 static void test_filestreams(skiatest::Reporter* reporter, const char* tmpDir) {
-    SkString path;
-    path.printf("%s%s", tmpDir, "wstream_test");
+    SkString path = SkOSPath::SkPathJoin(tmpDir, "wstream_test");
 
     const char s[] = "abcdefghijklmnopqrstuvwxyz";
 
@@ -98,8 +97,9 @@
     }
     delete[] dst;
 
-    if (!skiatest::Test::GetTmpDir().isEmpty()) {
-        test_filestreams(reporter, skiatest::Test::GetTmpDir().c_str());
+    SkString tmpDir = skiatest::Test::GetTmpDir();
+    if (!tmpDir.isEmpty()) {
+        test_filestreams(reporter, tmpDir.c_str());
     }
 }