Compiling import and export wrappers currently takes a big chunk of the
instantiation time. Add events to show this in the trace.

R=ahaas@chromium.org

Change-Id: Iffcabf18497cdb2d8caac7f939e4b1bc452d316f
Reviewed-on: https://chromium-review.googlesource.com/1148219Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54644}