Add `toolactivation` and `toolcancel` events [8/N]

This CL introduces two new DOM events, `toolactivation` and
`toolcancel`, which are dispatched on the `window` object when a WebMCP
tool starts execution and when it is cancelled, respectively.

Both events are of type `WebMCPEvent`, which includes a `toolName`
property identifying the tool involved.

Fixed: 481313974,475998009
Change-Id: I568dfcbf02882fee1856acc776ac2613935b80b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7540370
Reviewed-by: Ben Greenstein <bengr@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1579136}