Implement OnHidden and OnShown for UnstartedPagePaintObserver

When a navigation is started and before the first contentful paint the
page is hidden (because there was a tab switch or the browser window was
minimized), the navigation_timeout_timer_ keeps running potentially
expiring and reporting the histogram with Timeout set to true. However,
in some cases it may be that navigation is completed when the tab is
back to shown. In this case we should stop the time when the tab is
hidden (OnHidden) and start it in the case it is shown (OnShown) and the
first contentful paint was not yet received.

OBSOLETE_HISTOGRAM[PageLoad.Clients.NavigationToFirstContentfulPaint.Timeout]=Replaced by PageLoad.Clients.NavigationToFirstContentfulPaint.Timeout2

Change-Id: Ia6e1eb638e9bbcaea8a404cd94fbebc23214196a
Bug: 380411637
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6042605
Commit-Queue: Sergio Gonzalez Martin <sergiog@microsoft.com>
Reviewed-by: Andrew Williams <awillia@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1389298}