The EnableFlagScope is useful also for non-boolean flags. With the
template we can use if for example in the wasm fuzzers to reduce the
maximum memory size of a wasm module.

In addition I put the EnableFlagScope into the v8::internal namespace,
and I fixed a small typo.

BUG=v8:6474
R=clemensh@chromium.org

Change-Id: Iae5d5c058c334cd0f9e09d20adfd229fc2d6c585
Reviewed-on: https://chromium-review.googlesource.com/531005
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45862}