[gn] Turn on external_startup_data by default except on ios

This sets the default for the feature, as chromium expects
it: It is turned on for all platforms except ios.

Chromium's build_override can be removed after this.

This will also allow to override the value as a gn arg.

BUG=chromium:474921,chromium:616034
NOTRY=true

Review-Url: https://codereview.chromium.org/2025803003
Cr-Commit-Position: refs/heads/master@{#36666}

BUILD.gn

@@ -23,7 +23,7 @@ declare_args() {
 
   # Use external files for startup data blobs:
   # the JS builtins sources and the start snapshot.
-  v8_use_external_startup_data = true
+  v8_use_external_startup_data = !is_ios
 
   # Sets -DVERIFY_HEAP.
   v8_enable_verify_heap = false