Change defaults for build args to accommodate Flutter and Fuchsia.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 462aaa5420ef58d1345152cf0d3e290e5893b1fc..fc23609d09af00f7f71f812e6432e67df5723df6 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -196,6 +196,8 @@ if (current_os == "win") {
   is_win = false
 }
 
+is_flutter = false

[zra, 2017/08/04 22:40:41]

This is probably not the right place for this since Flutter isn't really an OS.
Instead, this should probably be a new GN argument under a new file, like
//build/flutter_args.gni (in case we need to add more Flutter-specific arguments
later.) Then you can import build/flutter_args.gni where you need is_flutter,
and not worry about saying (defined(is_flutter) && is_flutter) everywhere.

+
 # =============================================================================
 # BUILD OPTIONS
 # =============================================================================