[heap] Disable --scavenge-separate-stack-scanning flag by default

It was enabled accidentally.

Bug: chromium:1082426
Change-Id: I08019aeb3ddc34fea8ce9e08c6ace3a95335c171
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2199354
Auto-Submit: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67797}

src/flags/flag-definitions.h

@@ -909,7 +909,7 @@ DEFINE_BOOL(parallel_scavenge, true, "parallel scavenge")
 DEFINE_BOOL(scavenge_task, true, "schedule scavenge tasks")
 DEFINE_INT(scavenge_task_trigger, 80,
            "scavenge task trigger in percent of the current heap limit")
-DEFINE_BOOL(scavenge_separate_stack_scanning, true,
+DEFINE_BOOL(scavenge_separate_stack_scanning, false,
             "use a separate phase for stack scanning in scavenge")
 DEFINE_BOOL(trace_parallel_scavenge, false, "trace parallel scavenge")
 DEFINE_BOOL(write_protect_code_memory, true, "write protect code memory")