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: Idf29830a43f348fbf658fada0036c65ab410b155
Reviewed-on: https://chromium-review.googlesource.com/657397
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47931}

BUILD.gn

@@ -202,8 +202,12 @@ config("libsampler_config") {
 # This config should only be applied to code using V8 and not any V8 code
 # itself.
 config("external_config") {
+  defines = []
   if (is_component_build) {
-    defines = [ "USING_V8_SHARED" ]
+    defines += [ "USING_V8_SHARED" ]
+  }
+  if (v8_enable_v8_checks) {
+    defines += [ "V8_ENABLE_CHECKS" ]  # Used in "include/v8.h".
   }
   include_dirs = [
     "include",