[autotest] Fix chrome log collection

BUG=None
TEST=run desktopui_ChromeFirstRender (do "TESTS=desktopui_ChromeFirstRender emerge-x86-generic autotest autotest-tests" first, or you won't pick up the changes to site_ui_test.py!)

Change-Id: Icb366bbac87868eda3b14928058f1c4c8a631585

Review URL: http://codereview.chromium.org/3234004
2 files changed