Move v8_use_external_startup_data out of //build/config:feature_flags.

This CL removes the global define of V8_USE_EXTERNAL_STARTUP_DATA from
the list in //build/config:feature_flags, and updates the individual
call sites to use the definition as appropriate.

By making this change, //build/config will not need to import
//build_overrides/v8.gni, and that will make it easier for other
projects that use GN to use //build without needing to create a
v8 overrides file.

R=brettw@chromium.org
BUG=541791

Review URL: https://codereview.chromium.org/1413893003

Cr-Original-Commit-Position: refs/heads/master@{#355401}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a22b0730051957c6024527154919daf8de71afea

config/BUILD.gn

@@ -8,7 +8,6 @@ import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
-import("//build_overrides/v8.gni")
 import("//build/toolchain/goma.gni")
 
 declare_args() {
@@ -243,9 +242,6 @@ config("feature_flags") {
   if (enable_hangout_services_extension) {
     defines += [ "ENABLE_HANGOUT_SERVICES_EXTENSION=1" ]
   }
-  if (v8_use_external_startup_data) {
-    defines += [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
-  }
   if (enable_background) {
     defines += [ "ENABLE_BACKGROUND=1" ]
   }