Implement the full screen WebView plugin.

Use a NoTitleBar_Fullscreen dialog to implement the
full screen plugin. This runs in the same thread as
WebView (UI in the Browser case). One catch is that
the SurfaceView provided by the plugin needs to be
opaque if it doesn't want to see through the WebView.

The PluginFullScreenHolder translates the events to
the underline WebView. Special treatment in the touch
case as it needs to translate the coordinates.

WebView can't be panned, or double tap to zoom, or
long press to trigger the context menu while having
a full screen plugin.

Inside webkit, we also give the plugin element focus
when it goes to the full screen so that it takes key
events. While handling key events, we don't let it 
loose focus or scroll out.

Todo:
When a plugin goes to full screen, we should make
sure the embedded plugin is fully visible. Otherwise
when we translate the touch events back, they will be
outside of the visible rect and will be ignored.

This is part 1 of 2-project check in.