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Before this patch, the script executor checked element existence in
parallel and the script tracker checked script preconditions in
parallel.

This doesn't work: element existence checks only work when run one at a
time. This meant that precondition on elements were never met, because
there were always multiple scripts to check, while WaitForDomProto
worked, as it's typically running a check on a single element.

With this patch, checking for elements from preconditions works, even
when there are more than one script or elements to check.

Bug: 806868
Change-Id: Ib5e97a36a7aff5df5eab183da9461375df4f1ec3
Reviewed-on: https://chromium-review.googlesource.com/c/1256794Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596197}