Implement use_cfi_cast to optionally enable cast checks.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index e44570c706c612d1dbe7e9add24cd7d7272244c3..b06028be6a61a99db858616ea53b4f340074a531 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -1979,6 +1979,8 @@ test("base_unittests") {
     "win/wrapped_window_proc_unittest.cc",
   ]
 
+  defines = []
+
   deps = [
     ":base",
     ":i18n",
@@ -2009,7 +2011,7 @@ test("base_unittests") {
   # Allow more direct string conversions on platforms with native utf8
   # strings
   if (is_mac || is_ios || is_chromeos || is_chromecast) {
-    defines = [ "SYSTEM_NATIVE_UTF8" ]
+    defines += [ "SYSTEM_NATIVE_UTF8" ]
   }
 
   if (is_android) {
@@ -2114,6 +2116,11 @@ test("base_unittests") {
       # data += [ "$root_out_dir/base_unittests.dSYM/" ]
     }
   }
+
+  if (use_cfi_cast) {
+    # TODO(krasin): remove CFI_CAST_CHECK, see https://crbug.com/626794.
+    defines += [ "CFI_CAST_CHECK" ]
+  }
 }
 
 action("build_date") {