Otherwise the value cannot be used in gtests in EXPECT_GT expressions.
The reason is that EXPECT_GT takes values by reference and not by
value, so the declaration in v8.h is not enough.

R=clemensb@chromium.org

Bug: v8:9810
Change-Id: I6f85024d483701fcc65d1ccfc5254dbdbcce16c1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1871606Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64482}