[test] Restore "stress_incremental_marking" variant.

This removes the explicit --write-protect-code-memory flag from the list
of flags in the "stress_incremental_marking". The feature is enabled by
default by now and no longer needs explicit testing.

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

Change-Id: I5d6ba21dff261488bbe1b0148ce204bf78d57334
Reviewed-on: https://chromium-review.googlesource.com/860661Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50500}

tools/testrunner/local/variants.py

@@ -10,9 +10,7 @@ ALL_VARIANT_FLAGS = {
   "minor_mc": [["--minor-mc"]],
   "slow_path": [["--force-slow-path"]],
   "stress": [["--stress-opt", "--always-opt"]],
-  # 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"]],
+  "stress_incremental_marking":  [["--stress-incremental-marking"]],
   # 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.