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AXEventGenerator::Iterator iterates over the generated events in
std::map<AXNode*, std::set<EventParams>> tree_events_. Often there
could be entry with empty event sets (std::set<EventParams) in
|tree_events_|, and if these empty event sets happen to be the first
entry when we iterate over |tree_events_|, AXEventGenerator::Iterator
::operator++ would increment AXEventGenerator::Iterator::set_iter_
past its end, causing an exception.

This CL fixes this issue by looping over |tree_events_| in
AXEventGenerator::PostprocessEvents a second time to delete empty
empty std::set<EventParams> entry; so AXEventGenerator::Iterator
should only iterate over non-empty std::set<EventParams> entry.
DCHECKs are also added in AXEventGenerator::Iterator to check the
condition that only non-empty event sets remain.

AX-Relnotes: n/a

Bug: 1120281

Change-Id: I6c049be2cfb786c9777b4b840d598c77a8362e37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410925Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Victor Fei <vicfei@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#807115}