Fix checkdeps in third_party/WebKit.

Running checkdeps normally skips third_party subdirectories. However when uploading from third_party/WebKit, the presubmit checks run checkdeps on new include lines. I can't find a way to make checkdeps not ignore third_party/WebKit when checking the root src directory, so until we merge repos this might regress.

Review URL: https://codereview.chromium.org/231303002

git-svn-id: svn://svn.chromium.org/blink/trunk@171176 bbb929c8-8fbe-4397-9dbb-9b2b20218538