[heap] Enable concurrent marking for x86 and x64.

This is an experiment and will be reverted after getting canary
coverage.

Bug: chromium:694255
Change-Id: I40388d8c6db0e46e2ce64e88aba04c5ac8822e94
Reviewed-on: https://chromium-review.googlesource.com/625959
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47541}

BUILD.gn

@@ -78,7 +78,7 @@ declare_args() {
   v8_enable_trace_ignition = false
 
   # Sets -dV8_CONCURRENT_MARKING
-  v8_enable_concurrent_marking = false
+  v8_enable_concurrent_marking = ""
 
   # Sets -dV8_CSA_WRITE_BARRIER
   v8_enable_csa_write_barrier = false
@@ -144,6 +144,13 @@ if (v8_enable_v8_checks == "") {
 if (v8_check_microtasks_scopes_consistency == "") {
   v8_check_microtasks_scopes_consistency = is_debug || dcheck_always_on
 }
+if (v8_enable_concurrent_marking == "") {
+  if (v8_current_cpu == "x86" || v8_current_cpu == "x64") {
+    v8_enable_concurrent_marking = true
+  } else {
+    v8_enable_concurrent_marking = false
+  }
+}
 
 # Specifies if the target build is a simulator build. Comparing target cpu
 # with v8 target cpu to not affect simulator builds for making cross-compile