bug:3325039 Making the cursor ring movement send accessibility events.

1. Added a mechanism to select the cursor ring content if accessibility is
   enabled - This is achieved by sending an event to the WebCore thread
   (if accessibility is enabled) to select the content of the cursor when
   the latter moves. Added code in WebViewCore to select the given node
   and notify the UI thread for the selection markup which is delivered
   of the accessibility injector which manages sending accessibility
   events. This is relevant for adding accessibility to WebView if
   JavaScript is not enabled. (If JS is enabled we inject a screenreader
   written in JS).

2. Fixed the event delegation to the accessibility injector since it
   should be able to consume key events of interest and perform some
   action which leads to sending appropriate accessibility event.
   In the previous implementation it was possible that the injector
   consumes the event but the latter was bubbled up.

3. Added function to scroll the selection into view while moving it
   around based on user commands.

Note: This is a part of two project commmit.

Change-Id: Ibb81d0468726efbe3bf6e3add1b19c69e3206638