[test] Add testing of --write-protect-code-memory feature.

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}

tools/testrunner/local/variants.py

@@ -6,7 +6,9 @@
 ALL_VARIANT_FLAGS = {
   "default": [[]],
   "stress": [["--stress-opt", "--always-opt"]],
-  "stress_incremental_marking":  [["--stress-incremental-marking"]],
+  # TODO(6792): Write protected code has been temporary added to the below
+  # variant until the feature has been enabled (or staged) by default.
+  "stress_incremental_marking":  [["--stress-incremental-marking", "--write-protect-code-memory"]],
   # No optimization means disable all optimizations. OptimizeFunctionOnNextCall
   # would not force optimization too. It turns into a Nop. Please see
   # https://chromium-review.googlesource.com/c/452620/ for more discussion.
@@ -19,7 +21,9 @@ ALL_VARIANT_FLAGS = {
 FAST_VARIANT_FLAGS = {
   "default": [[]],
   "stress": [["--stress-opt"]],
-  "stress_incremental_marking":  [["--stress-incremental-marking"]],
+  # TODO(6792): Write protected code has been temporary added to the below
+  # variant until the feature has been enabled (or staged) by default.
+  "stress_incremental_marking":  [["--stress-incremental-marking", "--write-protect-code-memory"]],
   # No optimization means disable all optimizations. OptimizeFunctionOnNextCall
   # would not force optimization too. It turns into a Nop. Please see
   # https://chromium-review.googlesource.com/c/452620/ for more discussion.