[webview][profile][prefetch] Add More Perfetto Slices

This CL adds trace slices for code related to the WebView Profile
(i.e. the browser context) and prefetch. This is for the purpose of
improved debug-ability and observability for performance bottlenecks.

This feeds into our efforts to add public field tracing capabilities to
WebView.

Change-Id: I3aeb7aef13562b43a78f22ec4fc4b7d1baebef28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6448316
Reviewed-by: Bo Liu <boliu@chromium.org>
Commit-Queue: Wayne Jackson Jr. <wbjacksonjr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1448377}