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This page recently broke (fix in http://crrev.com/c/1880171), so tests
seem like a good idea.

This also adds a test helper for executing javascript, and makes the
NavigateAndWaitForCompletion function work more similarly to the one in
the instrumentation tests by waiting until the page is finished loading.

Bug: 1017924
Change-Id: I8ca048f9a9a92808abf6cfa5d0e091550cc3d9a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880173
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709552}