Cleanup: Rename SkOSPath functions.

Mostly for brevity and matches better with Python:

Python           | Old C++                 | New C++
os.path.join     | SkOSPath::SkPathJoin    | SkOSPath::Join
os.path.basename | SkOSPath::SkBasename    | SkOSPath::Basename

BUG=None
TEST=make all
R=mtklein@google.com, bsalomon@google.com

Author: tfarina@chromium.org

Review URL: https://codereview.chromium.org/428443002
diff --git a/dm/DM.cpp b/dm/DM.cpp
index 0077d5a..13079ca 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -128,8 +128,7 @@
     SkString filename;
     while (it.next(&filename)) {
         if (!SkCommandLineFlags::ShouldSkip(FLAGS_match, filename.c_str())) {
-            skps->push_back(
-                    SkOSPath::SkPathJoin(FLAGS_skps[0], filename.c_str()));
+            skps->push_back(SkOSPath::Join(FLAGS_skps[0], filename.c_str()));
         }
     }
 }
@@ -148,7 +147,7 @@
             exit(1);
         }
 
-        SkString filename = SkOSPath::SkBasename(skps[i].c_str());
+        SkString filename = SkOSPath::Basename(skps[i].c_str());
         tasks->add(SkNEW_ARGS(DM::SKPTask, (reporter, tasks, pic, filename)));
         tasks->add(SkNEW_ARGS(DM::PDFTask, (reporter, tasks, pic, filename,
                                             RASTERIZE_PDF_PROC)));