Add a new option to Tools/Scripts/run-webkit-tests (and WebKitTestRunner) to enable specifying webkit-test-runner Foo=bar header values for test runs — as if those values were provided in a “<!-- webkit-test-runner [ Foo=Bar ] -->” header within each test being run.

For example, given the following command-line invocation:

  run-webkit-tests --additional-header='useEphemeralSession=true' imported/w3c/web-platform-tests/clear-site-data/storage.https.html

…that invocation would cause the given test to be run in an ephemeral session — that is, as if the test itself had a “<!-- webkit-test-runner [ useEphemeralSession=true ] -->” header.