Move OnKillFocus() from OmniboxEditController to WebOmniboxEditController

The only usage of this method is in iOS-specific code that knows that it's
talking to a WebOmniboxEditController, so it's unnecessary to fork
omnibox_edit_controller.h to expose this method.

BUG=305330

Review URL: https://codereview.chromium.org/1465253002

Cr-Commit-Position: refs/heads/master@{#361105}

ios/chrome/browser/ui/omnibox/web_omnibox_edit_controller.h

@@ -18,6 +18,8 @@ class WebOmniboxEditController : public OmniboxEditController {
   // Returns the WebState of the currently active tab.
   virtual web::WebState* GetWebState() = 0;
 
+  virtual void OnKillFocus() = 0;
+
  protected:
   WebOmniboxEditController();
   ~WebOmniboxEditController() override;