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Adds a link-to-text button when clicking on selected text in the
browser container.
Creates and hooks up a mediator and tab helper to handle the click
on the new link-to-text button.

Bug: 1134703
Change-Id: I72196fc7a95b81ea32700414c70a645af16bfc52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446202
Commit-Queue: sebsg <sebsg@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814253}