AX: Make picker indicator elements focusable if accessibility is enabled.

We should provide a way to open calendar picker or datalist picker even if a
user doesn't know key bindings to open them.

This CL is not testable. We have no way to disable accessibility in layout
tests, and LayoutTestHelper::isRunningLayoutTest() returns true in C++ unit
tests too.

BUG=123896

Review URL: https://codereview.chromium.org/577553002

git-svn-id: svn://svn.chromium.org/blink/trunk@182041 bbb929c8-8fbe-4397-9dbb-9b2b20218538