Remove shouldPersist from WebPlugin.h.

The only caller of shouldPersist was removed in r201401. This removes the
method from the WebPlugin API.

BUG=370459

Review URL: https://codereview.chromium.org/1358123002

git-svn-id: svn://svn.chromium.org/blink/trunk/public@202635 bbb929c8-8fbe-4397-9dbb-9b2b20218538
diff --git a/web/WebPlugin.h b/web/WebPlugin.h
index 133a7fa..20aaf78 100644
--- a/web/WebPlugin.h
+++ b/web/WebPlugin.h
@@ -178,7 +178,6 @@
     virtual void rotateView(RotationType type) { }
 
     virtual bool isPlaceholder() { return true; }
-    virtual bool shouldPersist() const { return false; }
 
 protected:
     ~WebPlugin() { }