Win/GN: Make no_exception config explicit.

build/config/gcc/BUILD.gn

Index: build/config/gcc/BUILD.gn
diff --git a/build/config/gcc/BUILD.gn b/build/config/gcc/BUILD.gn
index 422ec0e92af9173212fe8eeec091a359f4f9b178..69425651fc5e9b371536efbd5874b4df28400bab 100644
--- a/build/config/gcc/BUILD.gn
+++ b/build/config/gcc/BUILD.gn
@@ -117,6 +117,7 @@ config("executable_ldconfig") {
   }
 }
 
+# TODO(jmadill): Remove this when ANGLE and libc++ don't depend on it.

[brettw, 2016/08/31 17:01:20]

I don't see how leaving this here helps ANGLE. It excepts to be able to remove
this from the list of default configs (removing something that isn't there is an
error), but you already removed it. Keeping this config here unreferenced
doesn't help those cases.

I don't know how much ANGLE needs this specific thing. If they can tolerate this
being wrong for a day, we should keep the reference in the default configs to
this one, but make this one empty. This way the GN angle code will run but it
will temporarily have no exceptions until it's updated to the shared one.

If that's not possible, we may need a 3-way patch. For a tricky case in the
past, I make a new .gni file in build somewhere with a bool set to false,
updated the other repo to key it's behavior off of the bool,  waited for a roll,
then update the behavior and flipped the bool, then removed everything.

 config("no_exceptions") {
   cflags_cc = [ "-fno-exceptions" ]
   cflags_objcc = cflags_cc