Introduce a new gyp flag to warn about to be deprecated APIs

build/features.gypi

Index: build/features.gypi
diff --git a/build/features.gypi b/build/features.gypi
index a3ee0fbbf08cd52e768984be9403ad8426c7eff3..cb69f880ef96cc7d7f7f94647764aa8ebfd21af9 100644
--- a/build/features.gypi
+++ b/build/features.gypi
@@ -59,6 +59,9 @@
     # Enable compiler warnings when using V8_DEPRECATED apis.
     'v8_deprecation_warnings%': 0,
 
+    # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
+    'v8_imminent_deprecation_warnings%': 0,
+
     # Set to 1 to enable DCHECKs in release builds.
     'dcheck_always_on%': 0,
   },
@@ -88,6 +91,9 @@
       ['v8_deprecation_warnings==1', {
         'defines': ['V8_DEPRECATION_WARNINGS',],
       }],
+      ['v8_imminent_deprecation_warnings==1', {
+        'defines': ['V8_IMMINENT_DEPRECATION_WARNINGS',],
+      }],
       ['v8_enable_i18n_support==1', {
         'defines': ['V8_I18N_SUPPORT',],
       }],