If an embedder doesn't re-use templates, we'll otherwise run out of
memory.

BUG=v8:6336
R=cbruni@chromium.org

Change-Id: Ib70054d3e9d12a6a606f0c4e35233edc69253c49
Reviewed-on: https://chromium-review.googlesource.com/493166Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45025}