Add path utils, plus a test for it.

Add path utils, plus a test for it.

SkOSFile:
Added class SkOSPath with functions for
modifying strings representing path names.

OSPathTest.cpp:
Test of the new utilities.

factory.cpp:
Use SkPathJoin.

gmmain and gm_expectations:
Use SkOSPath::SkPathJoin instead of a local version.

skimage_main.cpp:
Use the new location of SkPathJoin and SkBasename.

R=epoger@google.com

Review URL: https://codereview.chromium.org/15747004

git-svn-id: http://skia.googlecode.com/svn/trunk@9277 2bbb7eff-a529-9590-31e7-b0007b416f81