Relax Cast v8_use_external_startup_data req.

In preparation for turning it off on certain platforms

Bug: 1136105
Test: on affected device
Change-Id: I73ab150e54baba1e09e20559cd9c31500ceb3515
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466196Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Commit-Queue: Andrei Pascovici <apascovici@chromium.org>
Auto-Submit: Andrei Pascovici <apascovici@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816345}

chromecast/BUILD.gn

@@ -623,8 +623,6 @@ if (is_android) {
   }
 
   android_assets("cast_shell_apk_assets") {
-    assert(v8_use_external_startup_data)
-
     sources = [ "$root_out_dir/assets/cast_shell.pak" ]
 
     deps = [