During execution, the tier can be switch from Liftoff to TurboFan. We
already handle this in some locations by getting the tier before
execution, so that we later know which was the requested tier for that
unit.
In the case of the --wasm-tier-mask-for-testing flag, this accounting
was not right because the tier was already switched in the constructor.
This CL changes the compilation units to store both the requested and
the executed tier explicitly, so we know which counter to decrement
when the unit finishes.

R=ahaas@chromium.org

Bug: chromium:925671
Change-Id: I673463135e9b3ab17e40cfdfd5d3a526ad5a9b79
Reviewed-on: https://chromium-review.googlesource.com/c/1442639Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59168}