Fix scroll actions

- correct message string id (i.e. + '_description')
- walk up to the first scrollable ancestor (based on the availability of the scrolling action).

Test: 1. open Learn Mode. Perform three finger swipe up/down. Verify scroll back/forward output.
2. Scroll within an Android app. Verify content changes appropriately.

Change-Id: I4da02e1f09eb38dc2a61e5141b4ef67e57139b46
Reviewed-on: https://chromium-review.googlesource.com/1114200Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570326}

chrome/browser/resources/chromeos/chromevox/common/command_store.js

@@ -814,8 +814,8 @@ cvox.CommandStore.CMD_WHITELIST = {
       {announce: false, msgId: 'pause_all_media', category: 'information'},
 
   // Scrolling actions.
-  'scrollBackward': {msgId: 'action_scroll_backward'},
-  'scrollForward': {msgId: 'action_scroll_forward'},
+  'scrollBackward': {msgId: 'action_scroll_backward_description'},
+  'scrollForward': {msgId: 'action_scroll_forward_description'},
 
   // Math specific commands.
   'toggleSemantics':

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

@@ -722,10 +722,24 @@ CommandHandler.onCommand = function(command) {
         current = cursors.Range.fromNode(end);
       break;
     case 'scrollBackward':
-      current.start.node.scrollBackward();
+      var node = current.start.node;
+      while (node &&
+             !node.standardActions.includes(
+                 chrome.automation.ActionType.SCROLL_BACKWARD))
+        node = node.parent;
+
+      if (node)
+        node.scrollBackward();
       break;
     case 'scrollForward':
-      current.start.node.scrollForward();
+      var node = current.start.node;
+      while (node &&
+             !node.standardActions.includes(
+                 chrome.automation.ActionType.SCROLL_FORWARD))
+        node = node.parent;
+
+      if (node)
+        node.scrollForward();
       break;
     default:
       return true;