DSEPrewarm: Update TestNavigationObserver to ignore prerendering

This patch makes TestNavigationObserver ignore prerender navigation as
it can be non-deterministic, and can affect existing tests to make them
flaky or fail, especially when we support browser initiated
prerendering.

This change is needed to enable DSEPrewarm in the
fieldtrial_testing_config.json.

Change-Id: I57d832c3e9130be1ba6b57743c7ec6064014ecc5
Bug: 423465927
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6703920
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1482750}