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We have logic to iterate over all frames and wait for them to finish
loading, but that logic doesn't work if there are two frames with the
same url. Change the test to use frames with different urls.

Bug: 805998
Change-Id: I4d7d514cc0d638a95b2ddf445a7dbf59f1d73cef
Tbr: aboxhall@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/938028Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539288}