Add setting to isolate zoom changes by default.

content/browser/host_zoom_map_impl.h

Index: content/browser/host_zoom_map_impl.h
diff --git a/content/browser/host_zoom_map_impl.h b/content/browser/host_zoom_map_impl.h
index ad4c07e16c4247885814cbccfe96454f79fcfd05..d708fc288f547843d306560699c11b188e38b272 100644
--- a/content/browser/host_zoom_map_impl.h
+++ b/content/browser/host_zoom_map_impl.h
@@ -48,6 +48,8 @@ class CONTENT_EXPORT HostZoomMapImpl : public NON_EXPORTED_BASE(HostZoomMap),
                                     double level) override;
   bool UsesTemporaryZoomLevel(int render_process_id,
                               int render_view_id) const override;
+  double GetTemporaryZoomLevel(int render_process_id,
+                               int render_view_id) const override;
   void SetTemporaryZoomLevel(int render_process_id,
                              int render_view_id,
                              double level) override;
@@ -80,14 +82,6 @@ class CONTENT_EXPORT HostZoomMapImpl : public NON_EXPORTED_BASE(HostZoomMap),
                            double level,
                            const std::string& host);
 
-  // Returns the temporary zoom level that's only valid for the lifetime of
-  // the given WebContents (i.e. isn't saved and doesn't affect other
-  // WebContentses) if it exists, the default zoom level otherwise.
-  //
-  // This may be called on any thread.
-  double GetTemporaryZoomLevel(int render_process_id,
-                               int render_view_id) const;
-
   // Returns the zoom level regardless of whether it's temporary, host-keyed or
   // scheme+host-keyed.
   //