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One of the consequences of exposing Views as an AXTree is that
certain assumptions in AXPosition are no longer true.
One of those assumptions is that if a null position has been returned from a method
that moves from a non-null position to some other position, such as `CreatePositionAtEndOfAnchor`,
then the beginning or the end of the whole content has been reached.
This is no longer true when a tree of Views is connected to the main tree
representing the currently visible webpage.
It's also not true when AXPosition starts to be used more on Chrome OS, where the whole desktop
is exposed using a forest of AXTrees.

In this patch, the entirety of the AXPosition code was scanned for any assumptions that would
prevent it from working when the webpage's main tree is connected to a Views tree.
Also, care was taken to ensure that UI Automation text range provider methods work as before,
for e example navigating by word, line, etc. shouldnot stop at iframe boundaries.

AX-Relnotes: n/a.

Split out from patch that fixes CreatePositionAtStart/EndOfContent
https://chromium-review.googlesource.com/c/chromium/src/+/2595454/

Bug: 1049261
R=dmazzoni@chromium.org, aleventhal@chromium.org

Change-Id: I1a25d9f715c50773b418d322c2f7c52eacbd6732
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597568
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#839989}