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TestWebUIController is used by tests to create WebUIControllers with
specific CSPs and headers. Extract it into web_ui_browsertest_util.h
so that it can be shared by all WebUI tests.

Also rewrites tests to use the common TestWebUIController.

Bug: 1012150
Change-Id: I009c26534a7da37698c9a17ae22f2756ba3a4717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1921017
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716961}