Add early-outs in a couple of places after BrowserAddedWaiter usage.

This should get us better debug output when the BrowserAddedWaiter
times out. This is especially important before a following
install_observer.Wait() call, as otherwise the test will just get
killed during that wait, rather than printing the debug output that
is normally printed on test failure.

Change-Id: I668d3d75d736465619fd2c033b94366bcff8cd27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4351009


Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118931}