We should compute offscreen dynamically rather than getting it from Blink.

Let's fix  bug 764065  first, and then this one should be easy based on that one.

User-facing repro step with Switch Access:
1. Enable switch access in chrome://accessibility
2. Scroll the page up or down
3. Use the switches to try to navigate to an item that was previously offscreen -- you can't get to it.
3a. Use the switches to navigate to an item that was previously offscreen -- you can still get to it even though it is offscreen.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2c6052dbb3cbe85dbb310e7409686984a1fe67c2

commit 2c6052dbb3cbe85dbb310e7409686984a1fe67c2
Author: Katie Dektar <katydek@google.com>
Date: Wed Sep 27 00:32:32 2017

Set 'offscreen' state from bounds of node and ancestor.

Bug:  717677 
Change-Id: I68d78ede53492edf92e5a43182142b1d86b98a3a
Reviewed-on: https://chromium-review.googlesource.com/683298
Commit-Queue: Katie Dektar <katydek@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504537}
[modify] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/chrome/renderer/extensions/automation_internal_custom_bindings.cc
[modify] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/content/browser/accessibility/accessibility_tree_formatter_blink.cc
[modify] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/content/browser/accessibility/browser_accessibility.cc
[modify] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/content/browser/accessibility/browser_accessibility.h
[modify] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/content/browser/accessibility/dump_accessibility_tree_browsertest.cc
[add] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/content/test/data/accessibility/html/offscreen-expected-blink.txt
[add] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/content/test/data/accessibility/html/offscreen-scroll-expected-blink.txt
[add] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/content/test/data/accessibility/html/offscreen-scroll.html
[add] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/content/test/data/accessibility/html/offscreen.html
[modify] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/ui/accessibility/ax_tree.cc
[modify] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/ui/accessibility/ax_tree.h
[modify] https://crrev.com/2c6052dbb3cbe85dbb310e7409686984a1fe67c2/ui/accessibility/ax_tree_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b

commit bd85185f6c228a04882ad0aed1a44bdbd7dcb40b
Author: Katie Dektar <katydek@google.com>
Date: Fri Sep 29 04:44:25 2017

Removes Offscreen from internal ax states, leave in Automation.

Offscreen is now calculated separately and is no longer in the state
list.

Bug:  717677 
Change-Id: I6957d3de38e3b4c8356be1ff5f818c2c205ccc46
Reviewed-on: https://chromium-review.googlesource.com/688498
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505293}
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/chrome/renderer/extensions/automation_internal_custom_bindings.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/content/browser/accessibility/accessibility_tree_formatter_blink.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/content/browser/accessibility/browser_accessibility.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/content/browser/accessibility/browser_accessibility.h
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/content/browser/accessibility/browser_accessibility_manager_win.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/content/browser/accessibility/one_shot_accessibility_tree_search.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/content/browser/accessibility/one_shot_accessibility_tree_search_unittest.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/content/renderer/accessibility/blink_ax_enum_conversion.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/content/test/data/accessibility/html/input-text-value.html
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/accessibility/PRESUBMIT.py
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/accessibility/ax_enums.idl
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/accessibility/platform/ax_platform_node_delegate.h
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/accessibility/platform/ax_platform_node_win.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/accessibility/platform/ax_system_caret_win.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/accessibility/platform/ax_system_caret_win.h
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/accessibility/platform/test_ax_node_wrapper.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/accessibility/platform/test_ax_node_wrapper.h
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/views/accessibility/native_view_accessibility_auralinux.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/views/accessibility/native_view_accessibility_base.cc
[modify] https://crrev.com/bd85185f6c228a04882ad0aed1a44bdbd7dcb40b/ui/views/accessibility/native_view_accessibility_base.h

Google Chrome 64.0.3262.2 (Official Build) canary (64-bit)
Firmware Version Google_Samus.6300.174.0

I verified that this is fixed using the following steps: 
# Load any page, the example I used was chrome://settings/manageAccessibility
# Note that some items are offscreen, for example, mouse and touchpad and audio sections are offscreen 
# Scroll down to this offscreen area, bringing it onscreen 
# Select to speak an area of the screen such as the mouse and touchpad section. 
# Select to speak works as expected. 

Verified. 

Thanks,

Laura