The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/11805ff0b6aa419fec6e380a06402a39b3b13bb5

commit 11805ff0b6aa419fec6e380a06402a39b3b13bb5
Author: Dominic Mazzoni <dmazzoni@chromium.org>
Date: Thu Jun 21 23:11:09 2018

Free up accessibility tree resources from hidden frames.

When a frame is hidden, replace its accessibility tree with just the root,
and suppress all accessibility events in that tree while it's hidden.
By keeping the root in the tree, this allows us to propagate the change
to the browser process and free up resources there too.

The goal is that having lots of tabs open shouldn't significantly impact
your experience.

It may add a slight delay when switching to a tab.

Bug: 854411
Change-Id: I78b45e8f5772f0c9745ee59952f27f6ba3557bbc
Reviewed-on: https://chromium-review.googlesource.com/1107207
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569422}
[modify] https://crrev.com/11805ff0b6aa419fec6e380a06402a39b3b13bb5/chrome/test/data/extensions/api_test/automation/tests/tabs/tab_id.js
[modify] https://crrev.com/11805ff0b6aa419fec6e380a06402a39b3b13bb5/content/renderer/accessibility/blink_ax_tree_source.cc
[modify] https://crrev.com/11805ff0b6aa419fec6e380a06402a39b3b13bb5/content/renderer/accessibility/blink_ax_tree_source.h
[modify] https://crrev.com/11805ff0b6aa419fec6e380a06402a39b3b13bb5/content/renderer/accessibility/render_accessibility_impl.cc
[modify] https://crrev.com/11805ff0b6aa419fec6e380a06402a39b3b13bb5/content/renderer/accessibility/render_accessibility_impl.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a5955428b7ded84cc32618843735028e79d523d3

commit a5955428b7ded84cc32618843735028e79d523d3
Author: Dominic Mazzoni <dmazzoni@chromium.org>
Date: Tue Jul 10 20:09:53 2018

Revert: Free up accessibility tree resources from hidden frames.

It was causing too many problems like being unable to inspect background
tabs in chrome://accessibility and losing your place when switching tabs.
We should come up with a better fix.

Original change: crrev.com/c/1107207

TBR=nektar@chromium.org

Bug: 854411,  856163 ,  856160 
Change-Id: Iaef9b8ccbd47defba02e4e887f8219d00e77a808
Reviewed-on: https://chromium-review.googlesource.com/1131846
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573876}
[modify] https://crrev.com/a5955428b7ded84cc32618843735028e79d523d3/chrome/test/data/extensions/api_test/automation/tests/tabs/tab_id.js
[modify] https://crrev.com/a5955428b7ded84cc32618843735028e79d523d3/content/renderer/accessibility/blink_ax_tree_source.cc
[modify] https://crrev.com/a5955428b7ded84cc32618843735028e79d523d3/content/renderer/accessibility/blink_ax_tree_source.h
[modify] https://crrev.com/a5955428b7ded84cc32618843735028e79d523d3/content/renderer/accessibility/render_accessibility_impl.cc
[modify] https://crrev.com/a5955428b7ded84cc32618843735028e79d523d3/content/renderer/accessibility/render_accessibility_impl.h

Setting this aside for now

Downgrading P2 issues unmodified for more than 6 months, which have no owner or component.

Available, but no owner or component? Please find a component, as no one will ever find this without one.