This avoids generating redundant code for different template
instantiations.
I also introduce getters instead of accessing the fields directly.

R=ahaas@chromium.org
BUG=v8:6325

Change-Id: I3e0eca9ef6a01e0a3ebb73f4f357bcb59e120f43
Reviewed-on: https://chromium-review.googlesource.com/490166Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44976}