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In BrowserAccessibilityAndroid, the function HasFocusableNonOptionChild
is used to determine whether an accessibility node can be treated as a
leaf in IsLeaf. This function can be very costly (on the order of a few
milliseconds) since it traverses the tree.

IsLeaf is eventually called by both RetargetForEvent and CanFireEvents,
which in turn are called in every run of the loop over all events in
BrowserAccessibilityManager::OnAccessibilityEvents. For pages with a
complicated tree (e.g. cs.chromium.org), this can cause complete
unresponsiveness for periods of 10s and more.

This change moves the call to HasFocusableNonOptionChild behind the
individual checks for special situations in which the lack of focusable
children would allow to prune the accessibility tree. This results in
far less calls to the function while preserving the structure of the
tree.

Bug: 1014945
Change-Id: Idfde20271847425996d1f370e0fa1d8b964aa3f3
AX-Relnotes: n/a.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580802Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836650}