Add setting to isolate zoom changes by default.

content/public/browser/host_zoom_map.h

Index: content/public/browser/host_zoom_map.h
diff --git a/content/public/browser/host_zoom_map.h b/content/public/browser/host_zoom_map.h
index 61514db8d37da71b699d27d4e43604995fba30fe..27c232ff9158ef27bde9d3b5973bd60eb370b225 100644
--- a/content/public/browser/host_zoom_map.h
+++ b/content/public/browser/host_zoom_map.h
@@ -148,15 +148,23 @@ class HostZoomMap {
   virtual bool UsesTemporaryZoomLevel(int render_process_id,
                                       int render_view_id) const = 0;
 
-  // Sets the temporary zoom level that's only valid for the lifetime of this
-  // WebContents.
+  // Returns the temporary zoom level that's only valid for the lifetime of
+  // the given view (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.
+  virtual double GetTemporaryZoomLevel(int render_process_id,
+                                       int render_view_id) const = 0;
+
+  // Sets the temporary zoom level that's only valid for the lifetime of the
+  // given view.
   //
   // This should only be called on the UI thread.
   virtual void SetTemporaryZoomLevel(int render_process_id,
                                      int render_view_id,
                                      double level) = 0;
 
-  // Clears the temporary zoom level stored for this WebContents.
+  // Clears the temporary zoom level stored for the given view.
   //
   // This should only be called on the UI thread.
   virtual void ClearTemporaryZoomLevel(int render_process_id,