BrowserPlugin/WebView - Move plugin lifetime to DOM

This CL adds a mechanism whereby a plugin can have its lifetime managed within an HTMLPlugInElement, including early plugin creation (i.e. before it has been attached to the DOM and assigned a renderer), and persistent lifetime so that it is not killed if its renderer is deleted (e.g. display = 'none' is set on the element). The CL is motivated by the desire to make WebView a sutom element, though the approach used should extend to other plugins on an opt-in basis.

BUG=156219

Review URL: https://codereview.chromium.org/23618022

git-svn-id: svn://svn.chromium.org/blink/trunk@170791 bbb929c8-8fbe-4397-9dbb-9b2b20218538