[Extensions] Remove mojom::ActionType

mojom::ActionType is used in ExecuteCodeParams to indicate the type of
action that should be taken by an injection: kAddCss, kRemoveCss, and
kAddJavascript (there is no way to remove Javascript, naturally). Now
that there are dedicated types for CSS and JS injections, we can move
the add vs remove bit into the CSSInjection struct, and remove the
separate ActionType enum.

This CL should have no behavior change.

Bug: 1218648
Change-Id: I8afb10df45fda8483ea4b98bb47e0341ff47d997
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2961376
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#892379}