Get watermark feature ready for trusted testing.

Fix switching between tab when loading is slow.
Fix watermark appearing over ntp.
Fix lookups being made to chrome:// URLs during tab switches.

This CL includes a code to use the OnLoad signal to make sure that a
page that a page that should be watrmarked always is at the expense of
momentarily showing a watermark over a page that does not need one.
A better signal would be FCP, see b/330960313 for more details.

Change-Id: Ia0d9f20c6fd7a3f4c7e0378d5c07f9155c0fd4cf
Bug: b/328262616, b/329444366
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5356198


Reviewed-by: Nasser Al-shawwa <alshawwa@chromium.org>
Commit-Queue: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279215}