Allow embedder to set promise internal field count

gypfiles/features.gypi

Index: gypfiles/features.gypi
diff --git a/gypfiles/features.gypi b/gypfiles/features.gypi
index 684be69b6f2f55db091a286040709540430e799f..62c44bd9c93a3263c06f21bf7bc482aade8d4c01 100644
--- a/gypfiles/features.gypi
+++ b/gypfiles/features.gypi
@@ -31,6 +31,8 @@
   'variables': {
     'v8_enable_disassembler%': 0,
 
+    'v8_promise_internal_field_count%': 0,
+
     'v8_enable_gdbjit%': 0,
 
     'v8_enable_verify_csa%': 0,
@@ -77,6 +79,9 @@
       ['v8_enable_disassembler==1', {
         'defines': ['ENABLE_DISASSEMBLER',],
       }],
+      ['v8_promise_internal_field_count!=0', {
+        'defines': ['V8_PROMISE_INTERNAL_FIELD_COUNT','v8_promise_internal_field_count'],
+      }],
       ['v8_enable_gdbjit==1', {
         'defines': ['ENABLE_GDB_JIT_INTERFACE',],
       }],