[heap] Re-enable concurrent marking.

Bug: chromium:774644, chromium:694255
Change-Id: I957037b14bf6508e774d6fd1c97239b31f2296e8
Reviewed-on: https://chromium-review.googlesource.com/756893Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49187}

BUILD.gn

@@ -87,7 +87,7 @@ declare_args() {
   v8_enable_trace_feedback_updates = false
 
   # Sets -dV8_CONCURRENT_MARKING
-  v8_enable_concurrent_marking = false
+  v8_enable_concurrent_marking = true
 
   # Build the snapshot with unwinding information for perf.
   # Sets -dV8_USE_SNAPSHOT_WITH_UNWINDING_INFO.

gypfiles/features.gypi

@@ -85,7 +85,7 @@
     'v8_check_microtasks_scopes_consistency%': 'false',
 
     # Enable concurrent marking.
-    'v8_enable_concurrent_marking%': 0,
+    'v8_enable_concurrent_marking%': 1,
 
     # Controls the threshold for on-heap/off-heap Typed Arrays.
     'v8_typed_array_max_size_in_heap%': 64,