[gn] Use external startup data by default

BUG=chromium:474921
LOG=n
NOTRY=true

Review-Url: https://codereview.chromium.org/1971133002
Cr-Commit-Position: refs/heads/master@{#36210}

BUILD.gn

@@ -22,6 +22,10 @@ declare_args() {
   # http://v8project.blogspot.com/2015/09/custom-startup-snapshots.html
   v8_use_snapshot = true
 
+  # Use external files for startup data blobs:
+  # the JS builtins sources and the start snapshot.
+  v8_use_external_startup_data = true
+
   # Sets -DVERIFY_HEAP.
   v8_enable_verify_heap = false
 

build_overrides/v8.gni

@@ -9,7 +9,6 @@ if (is_android) {
   import("//build/config/android/config.gni")
 }
 
-v8_use_external_startup_data = false
 v8_optimized_debug = false
 v8_extra_library_files = []
 v8_experimental_extra_library_files = []