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/tools/filtermain.cpp b/tools/filtermain.cpp
index 0830c80..9df6c07 100644
--- a/tools/filtermain.cpp
+++ b/tools/filtermain.cpp
@@ -816,9 +816,9 @@
     if (iter.next(&inputFilename)) {
 
         do {
-            inFile = SkOSPath::SkPathJoin(inDir.c_str(), inputFilename.c_str());
+            inFile = SkOSPath::Join(inDir.c_str(), inputFilename.c_str());
             if (!outDir.isEmpty()) {
-                outFile = SkOSPath::SkPathJoin(outDir.c_str(), inputFilename.c_str());
+                outFile = SkOSPath::Join(outDir.c_str(), inputFilename.c_str());
             }
             SkDebugf("Executing %s\n", inputFilename.c_str());
             filter_picture(inFile, outFile);