Forward four more BrowserAccessibility APIs to AXPlatformNode.

put_accValue
put_accName
get_accHelpTopic
get_accValue

To support get_accValue, we extended AXPlatformNodeBase with a number of
helper methods (mostly copied from BrowserAccessibility).

BUG=703369

Review-Url: https://codereview.chromium.org/2933353002
Cr-Commit-Position: refs/heads/master@{#479946}
15 files changed