Add V8_ENABLE_CHECKS define to public config

We should add this define to external_config because it is used in
public include v8.h (e.g.: https://cs.chromium.org/chromium/src/v8/include/v8.h?l=272&rcl=5cd6565d5ad06a8cb5a1d9d502d15a54e4fa5bbe)

Change-Id: I795a3de448029e34033cf8f83094bdea3590bbb9
Reviewed-on: https://chromium-review.googlesource.com/654876Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47901}

BUILD.gn

@@ -205,6 +205,9 @@ config("external_config") {
   if (is_component_build) {
     defines = [ "USING_V8_SHARED" ]
   }
+  if (v8_enable_v8_checks) {
+    defines += [ "V8_ENABLE_CHECKS" ]  # Used in "include/v8.h".
+  }
   include_dirs = [
     "include",
     "$target_gen_dir/include",