Add beginnings of declarative WebMCP (1/N)

This adds the beginnings of a declarative WebMCP prototype, based on
this PR and comment:

- https://github.com/webmachinelearning/webmcp/pull/26/files
- https://github.com/webmachinelearning/webmcp/issues/22#issuecomment-3726418984

In this CL:
 - A <form> with *both* `toolname` and `tooldescription` attributes
   will trigger the addition of a new tool to the registry with that
   name and description.
 - ModelContext::ToolData now has a `declarative_tool` field, which
   holds callbacks for the tool itself and another to retrieve input
   schema when needed.
 - The input schema is updated each time the
   AIPageContentAgent::ContentBuilder::AddFrameData function is
   called. This is needed since it's possible the page gets updated
   in between calls.
 - When the tool is executed, for now, nothing happens other than a
   LOG() output.
 - The input schema is similarly hard-coded, and does not represent
   the actual DOM.
 - Much is left to do.

Change-Id: I12d0583f9c72bc72e4dbcea5ccd8534d61069919
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7428254
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1569476}