Add setting to isolate zoom changes by default.

content/browser/host_zoom_map_impl.cc

Index: content/browser/host_zoom_map_impl.cc
diff --git a/content/browser/host_zoom_map_impl.cc b/content/browser/host_zoom_map_impl.cc
index 97f15ddc7da338730a4a0f81b9d251b3f717f355..b21665ddadfd95acdd23eeb067c09f6c58d588db 100644
--- a/content/browser/host_zoom_map_impl.cc
+++ b/content/browser/host_zoom_map_impl.cc
@@ -143,6 +143,24 @@ void HostZoomMapImpl::CopyFrom(HostZoomMap* copy_interface) {
   default_zoom_level_ = copy->default_zoom_level_;
 }
 
+void HostZoomMapImpl::PreserveTemporaryZoomLevelFrom(
+    const HostZoomMap* copy_interface, RenderFrameHost* old_host,
+    RenderFrameHost* new_host) {
+  const HostZoomMapImpl* copy =
+      static_cast<const HostZoomMapImpl*>(copy_interface);
+  const RenderViewHost* old_view_host = old_host->GetRenderViewHost();
+  const RenderViewHost* new_view_host = new_host->GetRenderViewHost();
+
+  DCHECK(copy->UsesTemporaryZoomLevel(old_view_host->GetProcess()->GetID(),
+                                      old_view_host->GetRoutingID()));
+
+  const double level = copy->GetTemporaryZoomLevel(
+      old_view_host->GetProcess()->GetID(), old_view_host->GetRoutingID());
+  SetTemporaryZoomLevel(
+      new_view_host->GetProcess()->GetID(), new_view_host->GetRoutingID(),
+      level);
+}
+
 double HostZoomMapImpl::GetZoomLevelForHost(const std::string& host) const {
   base::AutoLock auto_lock(lock_);
   return GetZoomLevelForHostInternal(host);