[chromecast] Add AcceleratorAction to accessibility_private.json

Update chromecast's copy of accessibility private API. Add
AcceleratorAction to accessibility_private.json.

This is another candidate for duplicate code cleanup efforts.
(See crbug.com/837773)
However, leaving this in place for now.

This change is needed for downstream accessibility to work.

Test: build chromecast
Bug: None
Change-Id: I643642b821f247c928c6df27a338fb9b20d5b16b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416853Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Yuanyao Zhong <yyzhong@google.com>
Cr-Commit-Position: refs/heads/master@{#808370}

chromecast/common/extensions_api/accessibility_private.json

@@ -138,6 +138,12 @@
             "optional": true
           }
         }
+      },
+      {
+        "id": "AcceleratorAction",
+        "type": "string",
+        "enum": [ "focusPreviousPane", "focusNextPane" ],
+        "description": "A subset of accelerator actions used by accessibility."
       }
     ],
     "functions": [