Fix layout-tests-directory argument for devtools_web_tests command

Script chromium\src\third_party\blink\tools\blinkpy\web_tests\run_web_tests.py
defines an argument called --layout-tests-directory

However BUILD.gn uses it as --layout-test-directory (singular).

This, apparently, is leading to issues when trying to run devtools web tests
like here https://ci.chromium.org/p/devtools-frontend/builders/try/devtools_frontend_linux_blink_light_rel/7863?

Bug: 1137356
Change-Id: I01f247d786caf59ab0baef6fb881739e99b805f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466197Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#816232}

BUILD.gn

@@ -1189,7 +1189,7 @@ if (!is_ios) {
     script = _common_web_test_script
 
     args = _common_web_test_args + [
-             "--layout-test-directory=@WrappedPath(" + rebase_path(
+             "--layout-tests-directory=@WrappedPath(" + rebase_path(
                      "//third_party/devtools-frontend/src/test/webtests",
                      root_out_dir) + ")",
              "http/tests/devtools",