actions revamp: Enable context menu in extension popup

This CL adds the ability to trigger empty space / image / link context
menu in the extension action popup. The call to `ContextMenuCoordinator`
used to get squashed because we passed null for
`ContextMenuPopulatorFactory` when we attached WebContents to the
ThinWebView.

Demo: http://screenshot/9Q6XGi2L2GRPXaK

Bug: 429279387
Change-Id: I6d63416ed02ee5d3528c3f07eebafbad11c1c73d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7628018
Commit-Queue: Masa Fujita <massan@google.com>
Reviewed-by: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1594436}