[ios] Implement content shell menu item for toggling tracing

With this change, you can start and stop a trace with a hard-coded
set of categories. The resulting trace is accessible via the Files
app or, if running in the emulator, can be accessed via the path
that is written to the console.

Bug: 1411704
Change-Id: Ic732b2430cedec5d1bc3ed922c85f106ead79bab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4221796
Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1101562}