Also really useful for PDFs.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7d7a94d999847d71bc9f0557fd4103efa5a8ed71

commit 7d7a94d999847d71bc9f0557fd4103efa5a8ed71
Author: dmazzoni <dmazzoni@chromium.org>
Date: Mon Sep 26 20:22:47 2016

Implement NSAccessibilityScrollToVisibleAction on Mac

If you navigate to an element with VoiceOver, Chrome wasn't
scrolling it into view (unless it was focusable). To fix this
we mainly just needed to implement
NSAccessibilityScrollToVisibleAction.

To get this to work in PDF files too, we needed an extra fix
since the PDF node is outside of web content but can't scroll
itself, so we get the PDF node's coordinates and then tell the
web document to scroll to make that bounding box visible.

BUG= 649861 , 54724 

Review-Url: https://codereview.chromium.org/2365263003
Cr-Commit-Position: refs/heads/master@{#420977}

[modify] https://crrev.com/7d7a94d999847d71bc9f0557fd4103efa5a8ed71/content/browser/accessibility/browser_accessibility_cocoa.mm
[modify] https://crrev.com/7d7a94d999847d71bc9f0557fd4103efa5a8ed71/content/renderer/accessibility/render_accessibility_impl.cc
[modify] https://crrev.com/7d7a94d999847d71bc9f0557fd4103efa5a8ed71/content/renderer/accessibility/render_accessibility_impl.h
[modify] https://crrev.com/7d7a94d999847d71bc9f0557fd4103efa5a8ed71/third_party/WebKit/Source/modules/accessibility/AXObject.cpp