Make preferred color calculation consistent about which URL to use.

Code elsewhere uses main_frame_site.GetSiteURL(), but
OverrideWebPreferencesAfterNavigation() used GetLastCommittedURL(). This
disparity was introduced in https://crrev.com/c/2263635/15..16, with no
clear explanation as to why. While GetLastCommittedURL() is a plausible
value, if OverrideWebPreferences() is called subsequently it will
overwrite based on the site instance, so we should be consistent; and
based on the comments in https://crrev.com/c/1825936, we probably need
to compute the colors in cases where there is no last committed URL yet,
so it seems unsafe to use that value everywhere.

Bug: none
Change-Id: I1af43b30f13770c1ce3cee34c3b58abb3d2070c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6898316
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1508626}