Widen expectation for flaky progress events test

Unreviewed gardening. progress-events-generated-correctly.html
previously flaked only on win rel, but now also seen on linux and win
debug.

TBR=pfeldman@chromium.org
BUG=280342
NOTRY=true

Review URL: https://codereview.chromium.org/34583004

git-svn-id: svn://svn.chromium.org/blink/trunk@160230 bbb929c8-8fbe-4397-9dbb-9b2b20218538