Apply Data Controls policies based on the original page in Reading Mode for "Search with.." context menu item

When a context menu is opened within the Reading Mode side panel or
immersive view, Data Controls policies for features like "Search with"
should be evaluated against the URL of the original page being
distilled, not the Reading Mode's internal WebContents. This change
introduces a helper function to retrieve the correct WebContents for
Data Controls checks and updates the "Search with" logic to use it. New
browser tests are added to verify that Data Controls policies (block and
warn) are correctly applied in Reading Mode.

Bug: 509422826
Change-Id: I3d6e30c167905d64bb5aaa68458e4b3108d4f22b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7820234
Reviewed-by: Dominique Fauteux-Chapleau <domfc@chromium.org>
Commit-Queue: Ana Ercegovac <anaercegovac@google.com>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1626778}