[Watermark 2.0] Add Mojo interface to connect WebUI to backend.

This change introduces the necessary Mojo infrastructure to allow the
chrome://watermark WebUI page to communicate with the C++ browser
backend. Previously, the page was a visual-only tool with no way to send
its state to the browser.

The TypeScript frontend can now send the current font size and opacity
values to the backend via the `SetWatermarkStyle` method, that gets
called when the Copy To JSON button is clicked.

Bug: 434074405
Change-Id: I3cd5cbe11cb7cef3d1d44c072839c63770770830
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6792402
Reviewed-by: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Commit-Queue: Adam Kassi Lahlou <adamkl@google.com>
Cr-Commit-Position: refs/heads/main@{#1493686}