GTTF: Make runtest.py use --brave-new-test-launcher directly.
GTTF: Make runtest.py use --brave-new-test-launcher directly.

This is a preparation for at least two other changes:

1) Removing sharding_supervisor.py, which now is only a shim anyway.
2) Requesting JSON summary from the new test launcher.

BUG=236893

Review URL: https://codereview.chromium.org/56003007

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@233415 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build