Add GN arg to disable Alloy bootstrap (see #3681, see #3685)

Set enable_alloy_bootstrap=false to build with Alloy bootstrap code
removed. Extension API is documented as deprecated in comments but
not compiled out with this arg.

tests/ceftests/test_util.h

@@ -87,8 +87,10 @@ inline bool IsTestRequestContextModeCustom(TestRequestContextMode mode) {
 // Returns true if the old CefResourceHandler API should be tested.
 bool TestOldResourceAPI();
 
+#if !defined(DISABLE_ALLOY_BOOTSTRAP)
 // Returns true if the Chrome runtime is enabled.
 bool IsChromeBootstrap();
+#endif
 
 // Returns true if Views should be used as a the global default.
 bool UseViewsGlobal();