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..702d3d7aa5a1e115c47e7c1b40a8b0a2d522ae9e 100644
--- a/content/public/browser/host_zoom_map.h
+++ b/content/public/browser/host_zoom_map.h
@@ -19,6 +19,7 @@ namespace content {
 
 class NavigationEntry;
 class BrowserContext;
+class RenderFrameHost;
 class SiteInstance;
 class WebContents;
 
@@ -102,6 +103,12 @@ class HostZoomMap {
   // thread.
   virtual void CopyFrom(HostZoomMap* copy) = 0;
 
+  // Copy the temporary zoom level entry of |old_host| from the given map to
+  // this map's entry for |new_host|.
+  virtual void PreserveTemporaryZoomLevelFrom(
+      const HostZoomMap* copy, RenderFrameHost* old_host,
+      RenderFrameHost* new_host) = 0;
+
   // Here |host| is the host portion of URL, or (in the absence of a host)
   // the complete spec of the URL.
   // Returns the zoom for the specified |scheme| and |host|. See class
@@ -148,15 +155,15 @@ 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.
+  // 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,