Fix flakiness in AutomationApiTest.DesktopHitTestIframe

At least one issue was that the loadComplete event was sometimes being
received on a frame with an empty url.

Bug: 808343,622387
Change-Id: I7040a75c91f409ba07d3b9ad4e4598805e94b6ce
Reviewed-on: https://chromium-review.googlesource.com/938495
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540097}
3 files changed