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Previously, any time there was a layout, the accessibility
serialization code would scan every AXObject to see if any
needed to update their bounding box.

Instead, replace this with a mechanism that uses
PaintInvalidator to mark AXObjects that might have
dirty bounding boxes.

Existing tests already provide some coverage; if you
comment out the code in paint_invalidator.cc, a
handful of browser tests fail.

To provide even more coverage, a debug-only check
walks the entire tree and ensures that no nodes
have incorrect bounding boxes. If you try commenting
out the code in paint_invalidator.cc now, hundreds
of browser tests fail. This provides some good
confidence that it's working correctly.

Finally, add a new blink perf test demonstrating a
simple scenario where this results in a dramatic
speedup, just by focusing links in a document with a
few thousand nodes.

Bug: 1109081
AX-Relnotes: makes accessibility less sluggish on very large web pages

Change-Id: I51a89d0b37ff356c5443b324080acefe0e7f3fbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2319411
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792949}