Add CRWNativeContent willBeDismissed.

ios/web/web_state/ui/crw_web_controller_container_view.mm

Index: ios/web/web_state/ui/crw_web_controller_container_view.mm
diff --git a/ios/web/web_state/ui/crw_web_controller_container_view.mm b/ios/web/web_state/ui/crw_web_controller_container_view.mm
index 25e31bdead7566ca9822665fe69d06cb203c3764..fd97cd984375e8104b9bd89905247095968dee16 100644
--- a/ios/web/web_state/ui/crw_web_controller_container_view.mm
+++ b/ios/web/web_state/ui/crw_web_controller_container_view.mm
@@ -170,6 +170,9 @@
 }
 
 - (void)setNativeController:(id<CRWNativeContent>)nativeController {
+  if ([_nativeController respondsToSelector:@selector(willBeDismissed)]) {

[Eugene But (OOO till 7-30), 2016/12/21 15:26:35]

Should this be moved inside |(![_nativeController isEqual:nativeController])|
check? Otherwise |willBeDismissed| may be called even for cases when
nativeController is not changed, which does not sound right to me.

[Olivier, 2016/12/21 15:41:11]

The logic is to call in any navigation.
If the nativeContentProvider gives the same nativeController for two
navigations, it should be called twice. That is why I put it here.
I don't think it change a lot for OfflinePageNativeContent as BVC creates a new
one each time, so I can put it inside.

[Olivier, 2016/12/21 15:51:36]

Done.

+    [_nativeController willBeDismissed];
+  }
   if (![_nativeController isEqual:nativeController]) {
     base::WeakNSProtocol<id> oldController(_nativeController);
     [[oldController view] removeFromSuperview];