Reland "[heap] Enable concurrent marking for x86 and x64."

This is a reland of 8c4a8250
Original change's description:
> [heap] Enable concurrent marking for x86 and x64.
> 
> Bug: chromium:694255
> Change-Id: I28c8c6e5ba6c84123f3951e822c132860cb22c1d
> Reviewed-on: https://chromium-review.googlesource.com/641451
> Commit-Queue: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Reviewed-by: Hannes Payer (slow) <hpayer@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48100}

Bug: chromium:694255
TBR: mlippautz@chromium.org
Change-Id: Ic36515dbd418c219bccbbf371126a4dfd66a466f
Reviewed-on: https://chromium-review.googlesource.com/676966Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48107}

BUILD.gn

@@ -81,7 +81,7 @@ 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 = true
@@ -150,6 +150,13 @@ 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_target_cpu == "x86" || v8_target_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

gypfiles/features.gypi

@@ -82,8 +82,13 @@
     'v8_check_microtasks_scopes_consistency%': 'false',
 
     # Enable concurrent marking.
-    'v8_enable_concurrent_marking%': 0,
-
+    'conditions': [
+      ['v8_target_arch=="x64" or v8_target_arch=="ia32"', {
+        'v8_enable_concurrent_marking%': 1,
+      },{
+        'v8_enable_concurrent_marking%': 0,
+      }]
+    ],
     # Controls the threshold for on-heap/off-heap Typed Arrays.
     'v8_typed_array_max_size_in_heap%': 64,
   },