Always retry crashes and missing results when we retry failures in run-webkit-tests.

This makes the --retry-crashes flag a no-op that we can remove once
callers have been updated.

This change also removes the port.should_retry_crashes() and port.summarized_exit_code_from_results() methods as we no longer needed them, and removes the "num_missing" field from the full_results.json.

R=ojan@chromium.org
BUG=269204

Review URL: https://chromiumcodereview.appspot.com/22252005

git-svn-id: svn://svn.chromium.org/blink/trunk@155721 bbb929c8-8fbe-4397-9dbb-9b2b20218538