Forward BrowserAccessibility accSelect() to AXPlatformNode.

This forwards accSelect to the AXPlatformNode. In order to do this, we
had to extension AccessibilityPerformAction a bit to be able to handle
AX_ACTION_FOCUS. Not that the node id is not passed as part of the AXActionData
and is not being used by the delegate at this point.

BUG=703369

Review-Url: https://codereview.chromium.org/2913553003
Cr-Commit-Position: refs/heads/master@{#475806}
3 files changed