Detach web_state from DistillerPageiOS before destructing.

Follow up of https://chromium-review.googlesource.com/c/chromium/src/+/744012

Bug: 785673, 775684
Change-Id: I85e06ac43db3098ed4d297ea6ad9c242dd8ecae2
Reviewed-on: https://chromium-review.googlesource.com/774261Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517054}

components/dom_distiller/ios/distiller_page_ios.mm

@@ -121,7 +121,9 @@ bool DistillerPageIOS::StringifyOutput() {
   return false;
 }
 
-DistillerPageIOS::~DistillerPageIOS() {}
+DistillerPageIOS::~DistillerPageIOS() {
+  DetachWebState();
+}
 
 void DistillerPageIOS::AttachWebState(
     std::unique_ptr<web::WebState> web_state) {