[TestRunner] Fix ROOT_DIRECTORY path replacement on Windows
[TestRunner] Fix ROOT_DIRECTORY path replacement on Windows

This is blocking using the new test runner on Windows.

Bug: chromium:1186163
Change-Id: Ibba1b7f166e8ceb4c5008e712beb0c7f72bd75f2
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2828112
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Commit-Queue: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>
Auto-Submit: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Reviewed-by: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend