PrintTo for FilePath -- Pretty-print when comparing FilePaths in gtest

Used in upcoming http://codereview.chromium.org/10996005/ (and potentially others).

BUG=148539
TEST=Write a test with an error comparing two FilePaths and confirm the paths' values are printed by gtest as opposed to the byte-representation.


Review URL: https://chromiumcodereview.appspot.com/10984028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@158893 0039d316-1c4b-4281-b951-d872f2087c98