[ChildProcessLauncher] Set priority when receiving task port

On MacOS, the browser process can't directly set a renderer's priority
until the renderer has sent its task port to the browser. Until then,
calling `ChildProcessLauncher::SetProcessPriority` will have no effect,
effectively ignoring priority changes early in the process' lifetime.

With this change, `ChildProcessLauncher` will re-set the renderer's
priority when its task port is available, ensuring any previous calls to
`ChildProcessLauncher::SetProcessPriority` is correctly taken into
account.

Bug: 40067781
Change-Id: I238c72260700f5406fa73d3e17d7986e104efd46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6482458
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457367}