Stop using Chrome's automation extension for testharness tests #6348
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This prevents ChromeDriver from using its automation extension for
testharness-based tests. When this extension is used, Chrome displays
a warning bar that changes the window size while a test is already
running, and this can cause test flake (IntersectionObserver tests
are running into this; also see this discussion thread).
This change only affects testharness-based tests since older versions
of Chrome (like the currently shipping Chrome 59) require the extension
for reftests (in order to set the window size).
This change is