run-webkit-test should look in --root directory for LayoutTestRelay

run-webkit-test should look in --root directory for LayoutTestRelay
https://bugs.webkit.org/show_bug.cgi?id=150859

Reviewed by Daniel Bates.

For iOS run-webkit-tests, use LayoutTestRelay specified by --root; otherwise find
LayoutTestRelay in the Mac build directory when --root is unspecified.

* Scripts/webkitpy/port/base.py:
(Port._build_path): Use '_cached_root' instead of 'root' so that we don't overwrite the
argument that was passed in via the --root argument.
* Scripts/webkitpy/port/ios.py:
(IOSSimulatorPort.relay_path):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@192087 268f45cc-cd09-0410-ab3c-d52691b4dbfc