[chromeos] Make the doc viewing test more robust

[chromeos] Make the doc viewing test more robust

BUG=143302
TEST=desktopui_DocViewing should be green in chromeos BVT

R=craigdh@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=152005

[Nirnimesh, 2012-08-17 00:04:26]

I've been unable to see the flake locally, but hopefully it makes the test more
robust on the BVT runs.

[craigdh, 2012-08-17 00:10:04]

lgtm.