[Code Health] Migrate to base::Value::Dict& in webrequest_action

This adds a ValueT template parameter to DedupingFactory so that a
concrete type can be used throughout. The only other user of
DedupingFactory is WebRequestConditionAttribute, which could undergo the
same (using base::Value::List&), but I did not include that in this change.

Bug: 1315843
Change-Id: I074cb38ea76025e969f59011532b28cb8ad3ebd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3602974


Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#995404}