This makes reopening the handle easier, and also reduces the size of
the handle space.

Drive-by change: Make all fields of the ModuleCompiler private.

R=clemensh@chromium.org

Change-Id: I2f9553a00012b9ce7743bf68bdf1c3095ca69fe0
Reviewed-on: https://chromium-review.googlesource.com/548397
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46255}