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

This reverts commit 8bbc2242.

Reason for revert: On Canary 3195.

Original change's description:
> [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}

TBR=ulan@chromium.org,mlippautz@chromium.org

Change-Id: I642c1f778267a795bf1e1a6bba863552394ad1d4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:694255
Reviewed-on: https://chromium-review.googlesource.com/631717Reviewed-by: Michael Hablich <hablich@chromium.org>
Commit-Queue: Michael Hablich <hablich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47564}

BUILD.gn

@@ -78,7 +78,7 @@ 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 = false
@@ -144,13 +144,6 @@ 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_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