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This is a follow-up to http://crrev.com/c/2426490 which prevents an
infinite loop in AXPosition::AsValidDOMPosition by ensuring we have
a changed AXPosition after calling CreatePreviousPosition in generated
content. That fix failed to check that the previous position created
was not null prior to seeing if it had indeed changed.

Bugs: 1131019, 1133124

AX-Relnotes: Prevents a page crash when accessibility is enabled.
Change-Id: Iaa8a7d79c5750809e6a3d39f7077f6593f95488d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440630
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812793}