Fix broken build for certain flags combos

As part of enabling the flag described in the bug, I realize now that
certain Android builds of Chrome use the gn flags listed in Test: which
are broken.

Bug: 1136105
Test: gn gen //content/test:test_support --check
--args='v8_use_external_startup_data=false symbol_level=1
is_component_build=false' actually works with this, broken w/o


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: I8cb9f60f45d0d3d8cdbcdf44df10185522932475
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2483204Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Andrei Pascovici <apascovici@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818887}

content/public/test/browser_test_base.cc

@@ -94,7 +94,7 @@
 #include "ui/base/ui_base_paths.h"
 
 #ifdef V8_USE_EXTERNAL_STARTUP_DATA
-#include "gin/v8_initializer.h"
+#include "gin/v8_initializer.h"  // nogncheck
 #endif
 #endif