Add setting to isolate zoom changes by default.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index aa9fbc0bcc86a95f72e76dae88efd37d8dfeca5f..e7975ea41f8dee5f4133b5cf5bba9afd9466fdf4 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -910,6 +910,11 @@ void WebContentsImpl::ClosePage() {
   GetRenderViewHost()->ClosePage();
 }
 
+void WebContentsImpl::OnReplaced(WebContents* new_contents) {
+  for (auto& observer : observers_)
+    observer.WebContentsReplaced(this, new_contents);
+}
+
 RenderWidgetHostView* WebContentsImpl::GetRenderWidgetHostView() const {
   return GetRenderManager()->GetRenderWidgetHostView();
 }