The FuzzerSupport was keeping a single instance of itself. With this CL,
this instance is now stored in a unique_ptr. Therefore it is not
necessary to register an onExit callback to delete the FuzzerSupport
instance.

Drive-by changes: Some cleanup with the FuzzerSupport.

R=clemensh@chromium.org

Bug: chromium:787723
Change-Id: I5188c7aa7e778ccd45fc80ed0115c947d23a0dee
Reviewed-on: https://chromium-review.googlesource.com/792949Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49723}