Re-add the path() method of WebKitPatch.

The path() method was removed in http://crrev.com/2110893003 because I
thought it was unnecessary; it turns out it's used by rebaseline commands
to construct the command used to invoke other commands.

BUG=621181

Review-Url: https://codereview.chromium.org/2137013004
Cr-Commit-Position: refs/heads/master@{#404747}
3 files changed