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When the previous view controller's title is too long, iOS may choose to
label the navigation bar's back button with either the word "Back" or to
omit the label entirely. Have EG tests search for the back button using
all three potential labels.

BUG=863476

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ic7cae966be78e63b4a26f0e2d2a1f708b43a46d0
Reviewed-on: https://chromium-review.googlesource.com/1148466
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577649}