The path used by that option only comes into play when default parameters
are allowed but destructuring assignment is disallowed. Removing it
allows the removal of one implementation of ParseExpression, and makes
it clearer which code will be dead once all the destructuring flags
are removed.

Also made the |flags| param strongly typed instead of an int.

Review URL: https://codereview.chromium.org/1691653002

Cr-Commit-Position: refs/heads/master@{#33918}