Implement support for closing shadow DOM plugin placeholders.

This is based on the current heuristic in
plugins::PluginPlaceholder::HidePlugin
(components/plugins/renderer/plugin_placeholder.cc).

Tests are added to ensure that the placeholder closing code works as
expected, and to verify that the newly-added WebPluginPlaceholder method
is respected.

A crude UI is currently used; showing the X graphics currently used is blocked
on deciding how to load image resources in this environment.

BUG=364716

Review URL: https://codereview.chromium.org/698533003

git-svn-id: svn://svn.chromium.org/blink/trunk@184852 bbb929c8-8fbe-4397-9dbb-9b2b20218538