Add the aria-readonly attribute to the x-screen element
Add the aria-readonly attribute to the x-screen element

This adds the aria-readonly attribute to x-screen element. Currently
on ChromeOS ChromeVox sends Search+Arrow key combinations through to
contenteditable regions as Up/Down/PgUp/PgDown keys. This works well for
normal contenteditable regions as it allows the cursor to be moved
through the editable region and when it reaches the top/bottom of the
region, ChromeVox will continue navigating through the rest of the
elements of the page. However in the case of hterm, the contenteditable
region is not really editable - the cursor is hidden and input events
(in particular PgUp/PgDown/Home/End) are consumed and preventDefaulted.
This results in element navigation not working well in ChromeVox.

Marking the region as aria-readonly gives ChromeVox an indication that
the content is actually not modifiable.

Bug: 822490
Change-Id: I74c3cb7e4730914c06ef5b0991225cc03072578a
Reviewed-on: https://chromium-review.googlesource.com/1009245
Reviewed-by: David Tseng <57d6711e15f60434c9e159ea3db9eda3f784ef2f@chromium.org>
Reviewed-by: Vitaliy Shipitsyn <6f51537d61e46a00bf958b60dbef5b5870c2bf22@google.com>
Tested-by: Raymes Khoury <348ad72dc039a127ca70ef38e5de4eebe052b884@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm