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Much more reliable capture based on a state machine. Now thumbnail
capture walks through the following steps:
 - Navigating to new page
 - Navigation complete
 - Capture requested
 - Frame capture
 - Cooldown*
 - Capture complete**

 * Cooldown allows us to continue capturing for a short period of time
   after a page indicates it's loaded; this gives the renderer time to
   actually draw the final version of the page
** Once we've captured a background page, there's no further need to try
   to capture it again unless it navigates, or the user makes it active,
   interacts, and then leaves again (a case we handle separately).

Typically these states move forward. However, there are potential issues
with trying to frame capture a visible/active tab, so we pause video
capture while a page is active and then return to capturing it after the
user switches away. Since we do not show preview images for the current
tab in desktop mode, it's not a problem. For tablet mode (Mohnstrudel),
if the tabstrip is observing the thumbnail image for the current tab,
the tab will be captured.

This complex behavior of stepping forward and backwards based on whether
a tab is visible, being observed, etc. is all handled by
ThumbnailTabHelper::ThumbnailTabHelperState::UpdateCaptureState().

Still to do:
 - Create a queue of thumbnails to be captured that prioritizes
   thumbnails for pages that are being or have recently been observed,
   to actively limit the number of thumbnails we are trying to observe
   at once in order to save memory. In-flight thumbnails are
   uncompressed, which can have a significant impact on RAM usage.

Bug: 1057713, 1059862, 1068459
Fixes: 1068459
Change-Id: If81798c0d3df26a5c309e834483042a5b5f24857
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145161
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761111}