Make WPT's .gitignore only ignore the testharness_runner.html in the root dir.

This avoids ignoring $wpt/tools/wptrunner/wptrunner/testharness_runner.html, which
is auto-imported and required to run WPT on Chromium CI.

Bug: 937369
Change-Id: I75a15f7e848ffc50df9e38f1c2cfb5b5ac110358
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1527876
Auto-Submit: Luke Z <lpz@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641609}
1 file changed