Fix delete key with ContentEditable fields.

 Bug 5806859
 Bug 5795341
 Add a custom InputConnection to capture the deleteSurroundingText
 command and use it to send delete key codes to the WebView for
 single deletes or calling into WebViewCore to delete contents for
 more complex changes.
 Also makes the IME options "normal text" for webkit-based text
 entry.
 Webkit change: Ic938d6fa

Change-Id: I18606014a034194928a61981dc3e2400a7db5fac