[heap] Temporarily disable concurrent marking.

The 6.3 branch has a data race that is fixed in 6.4 but the fix is too
large for back merging.

This CL will be back-merged to 6.3 after getting Canary coverage.

Concurrent marking will be re-enabled afterwards.

Bug: chromium:774644
Change-Id: I4112da0e133a637cc4fb52dee2e4c165cdc74f1f
Reviewed-on: https://chromium-review.googlesource.com/749811Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49080}

BUILD.gn

@@ -87,7 +87,7 @@ declare_args() {
   v8_enable_trace_feedback_updates = false
 
   # Sets -dV8_CONCURRENT_MARKING
-  v8_enable_concurrent_marking = true
+  v8_enable_concurrent_marking = false
 
   # Sets -dV8_CSA_WRITE_BARRIER
   v8_enable_csa_write_barrier = true

gypfiles/features.gypi

@@ -85,7 +85,7 @@
     'v8_check_microtasks_scopes_consistency%': 'false',
 
     # Enable concurrent marking.
-    '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,