Uses OnTreeDataChanged event throughout the codebase

The AXTreeData might change during an update to the tree and an
`OnTreeDataChanged` event will be fired.
This patch ensures that the right event is utilized in BrowserAccessibilityManager,
whenever the tree data changes. It also makes a copy of the new tree data for use
in followup patches in order to determine the ignored state of nodes.
`AXTree::UpdateTreeData` is changed to indicate that it should only be used for testing
and it is streamlined by having it call `AXTree::Unserialize` instead of implementing its own
logic.
In this way, if we ever change how updates to the tree data are treated,
we won't need to modify `AXTree::UpdateDataForTesting` as well.

R=dmazzoni@chromium.org, aleventhal@chromium.org

AX-Relnotes: n/a.
Change-Id: I36366d225c07c77c0f121418fa10feef92ab2aa5
Bug: 1049261
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2903292
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#888535}