Allow braille cursor routing at the end of the displayed braille content

Test: press braille cursor routing key one characer beyond the last character of the displayed text. See action applied to the end of the content such as selection.
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I50a6e5f1f1d708595ad52d026bf279267c96a233
Reviewed-on: https://chromium-review.googlesource.com/599108
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491610}

chrome/browser/resources/chromeos/chromevox/cvox2/background/background.js

@@ -634,7 +634,14 @@ Background.prototype = {
   brailleRoutingCommand_: function(text, position) {
     var actionNodeSpan = null;
     var selectionSpan = null;
-    text.getSpans(position).forEach(function(span) {
+
+    // For a routing key press one cell beyond the last displayed character, use
+    // the node span for the last character. This enables routing selection to
+    // the length of the text, which is valid.
+    var nodePosition = position;
+    if (position == text.length && position > 0)
+      nodePosition--;
+    text.getSpans(nodePosition).forEach(function(span) {
       if (span instanceof Output.SelectionSpan) {
         selectionSpan = span;
       } else if (span instanceof Output.NodeSpan) {