Revert "Cleanup: Remove launched CaptureThumbnail feature checks"

This reverts commit f9b5f09729c9d0538fc0e7b6f5d8ff723b0aaf2a.

Reason for revert: crbug.com/774798 - let's keep the feature checks
around for now.

Original change's description:
> Cleanup: Remove launched CaptureThumbnail feature checks
> 
> CaptureThumbnailDependingOnTransitionType and CaptureThumbnailOnNavigatingAway
> have been launched. This CL removes the feature checks.
> 
> Bug: 765588
> Change-Id: I68906ef0533ede82dc3c47724603b9af78494969
> Reviewed-on: https://chromium-review.googlesource.com/684839
> Reviewed-by: Friedrich Horschig <[email protected]>
> Commit-Queue: Marc Treib <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#508722}

[email protected],[email protected]

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 765588
Change-Id: Iaab1b35b3fb00e817e3311e11fffa3a23553ef99
Reviewed-on: https://chromium-review.googlesource.com/723384
Reviewed-by: Marc Treib <[email protected]>
Reviewed-by: Friedrich Horschig <[email protected]>
Commit-Queue: Marc Treib <[email protected]>
Cr-Commit-Position: refs/heads/master@{#509444}
9 files changed