Fix document leak from context menu.

A context menu hold a reference to the target Node in two places, ContextMenuController and RenderViewImpl.
This is not cleared until the next context menu is opened, causing a leak.

This patch tries to fix the bug by explicitly clearing the references when document is detached. The reference on ContextMenuController is cleared by Page::documentDetached, and the reference on RenderViewImpl is cleared by WebViewClient::clearContextMenu() interface. The RenderViewImpl::clearContextMenu implementation is to be added by http://crrev.com/22997003 .

BUG=270000

Review URL: https://chromiumcodereview.appspot.com/23012002

git-svn-id: svn://svn.chromium.org/blink/trunk@156017 bbb929c8-8fbe-4397-9dbb-9b2b20218538