This adds test coverage for write-protected code pages by adding the
feature to the existing "stress_incremental_marking" variant. Since this
is considered a temporary state, the variant has not been renamed.

R=machenbach@chromium.org
BUG=v8:6792

Change-Id: I62fa5aa15bf5309b12fbc269fcf81069845755f9
Reviewed-on: https://chromium-review.googlesource.com/753388Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49267}