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

This reverts commit 8c4a8250.

Reason for revert: Flaky dcheck on several bots, e.g.:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20debug/builds/17055

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}

TBR=ulan@chromium.org,haraken@chromium.org,machenbach@chromium.org,hpayer@chromium.org,mlippautz@chromium.org

Change-Id: Id5954676c75e69b66e85f05ffab737ab7f760101
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:694255
Reviewed-on: https://chromium-review.googlesource.com/677203Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48103}

BUILD.gn

@@ -81,7 +81,7 @@ declare_args() {
   v8_enable_trace_ignition = false
 
   # Sets -dV8_CONCURRENT_MARKING
-  v8_enable_concurrent_marking = ""
+  v8_enable_concurrent_marking = false
 
   # Sets -dV8_CSA_WRITE_BARRIER
   v8_enable_csa_write_barrier = true
@@ -150,13 +150,6 @@ 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

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