Die without a g_browser_process in TestingProfile

This provides a better error message (than nullptr dereference) in
TestingProfile. This would've helped when trying to figure out why an
EXPECT_CHECK_DEATH() was failing (no TestingBrowserProcess set up in the
death "fork", died for the wrong reason).

Bug: 332587367
Change-Id: I8d7ed71ed4cdce7b940ff4b29ea34fb5f849ed4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5422030


Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Auto-Submit: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282198}