This CL implements support for file: include lines in OWNERS files,
both as top-level directives and as per-file directives. The
paths can be either relative or absolute.

Examples of lines in OWNERS files:

  file:test/OWNERS  (relative, top-level)
  file://content/OWNERS  (absolute, top-level)
  per-file mock_impl.h=file:test/OWNERS  (relative, per-file)
  per-file mock_impl.h=file://content/OWNERS  (absolute, per-file)

A whole series of tests to cover this feature have been added
to owners_unittest.py as well.

BUG=119396, 147633

Review URL: https://codereview.chromium.org/1085993004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294854 0039d316-1c4b-4281-b951-d872f2087c98