Marking shared functions for tier-up was optimizing the functions
non-concurrently, to avoid the case where the same shared function is
optimized concurrently by multiple JS functions. This was particularly a
problem for small functions, which (if called in a loop) could get
marked for optimisation quite quickly.

In this CL, the shared function is instead marked as having an active
optimization job running, and these do not spawn a compilation job.

BUG=chromium:693590
BUG=chromium:700863
BUG=chromium:701665

Change-Id: I2b1c5af8e7aa8d779f86814c22c65c78bee0630f
Reviewed-on: https://chromium-review.googlesource.com/455779Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43833}