[build] Use deprecation flags in code importing v8

Add v8_deprecation_warnings and v8_imminent_deprecation_warnings to
external_config so they can also control how embedders see deprecation
warnings.

Also defaults v8_deprecation_warnings to true.

Change-Id: I267607d7b3e1ff6ee34a5555722118f9f22e0791
Reviewed-on: https://chromium-review.googlesource.com/1151110Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54715}

BUILD.gn

@@ -39,7 +39,7 @@ declare_args() {
   v8_enable_verify_predictable = false
 
   # Enable compiler warnings when using V8_DEPRECATED apis.
-  v8_deprecation_warnings = false
+  v8_deprecation_warnings = true
 
   # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
   v8_imminent_deprecation_warnings = true
@@ -261,6 +261,12 @@ config("external_config") {
   if (v8_enable_v8_checks) {
     defines += [ "V8_ENABLE_CHECKS" ]  # Used in "include/v8.h".
   }
+  if (v8_deprecation_warnings) {
+    defines += [ "V8_DEPRECATION_WARNINGS" ]
+  }
+  if (v8_imminent_deprecation_warnings) {
+    defines += [ "V8_IMMINENT_DEPRECATION_WARNINGS" ]
+  }
   include_dirs = [
     "include",
     "$target_gen_dir/include",