[heap] Test enabling of FLAG_local_heaps by default

Experiment with enabling FLAG_local_heaps for a few days to see whether
pause times regress and to catch bugs.

Bug: v8:10315
Change-Id: Iefc329bbc1b8131796f1db2a729ee358bef88894
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2375190Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69561}

src/flags/flag-definitions.h

@@ -952,7 +952,7 @@ DEFINE_BOOL(concurrent_marking, V8_CONCURRENT_MARKING_BOOL,
 DEFINE_BOOL(concurrent_array_buffer_sweeping, true,
             "concurrently sweep array buffers")
 DEFINE_BOOL(concurrent_allocation, false, "concurrently allocate in old space")
-DEFINE_BOOL(local_heaps, false, "allow heap access from background tasks")
+DEFINE_BOOL(local_heaps, true, "allow heap access from background tasks")
 DEFINE_IMPLICATION(concurrent_inlining, local_heaps)
 DEFINE_BOOL(stress_concurrent_allocation, false,
             "start background threads that allocate memory")