android: Add CCT test in StartupLoadingMetricsTest

The new test loads a page in CCT, verifies that it does not cause
startup metrics to be recorded (as intended). Then it loads another page
in CCT and checks that the metrics are still not recorded.

The second URL load in the test requires updating the
PageLoadMetricsTest. This is because load metrics observers in Java are
not tied to a single navigation as in C++. Instead of that they receive
all metrics events as long as they are registered in a global observer
list. The change is to remove the assertion checking that WebContents
remains the same across the test. See crrev.com/521333.

Bug: 1471136
Change-Id: I143bbd686ec3755b9b9bc27dc6bcb8c4be40eb98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4927308
Commit-Queue: Egor Pasko <pasko@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1207818}