[telemetry] Attempt to fix testActivateTab.
[telemetry] Attempt to fix testActivateTab.

The test immediately checks for document visible. It might not be true because the page hasn't finished loading yet. So we should just wait for it.


BUG=326725
TEST=tools/telemetry/run_tests testActivateTab  # Though this isn't a 100% test, since the flakiness was rare and Windows-only.
TBR=tonyg

Review URL: https://codereview.chromium.org/110623002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@241193 0039d316-1c4b-4281-b951-d872f2087c98

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