[webview][prefetch][2/n] - Add & Wire Prefetch Started Browser
Callback

This CL adds the "prefetch started" browser callback that is called
once the prefetch request initially kicks off. This callback is not
called at all for any redirects caused by the initial prefetch.

WebView will use this callback to resolve the `Future` returned
by the application's call to start the prefetch via. the WebView
prefetch API.

Bug: 363946909
Change-Id: I03c68418c401dce53b692a39510ad246bb81ddb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5829415
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Wayne Jackson Jr. <wbjacksonjr@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1355842}