TestExpectation: revive unexpected entries deletion

It's too brave to remove all entries...

TBR=morrita@chromium.org
BUG=133093

Review URL: https://codereview.chromium.org/20544005

git-svn-id: svn://svn.chromium.org/blink/trunk@155332 bbb929c8-8fbe-4397-9dbb-9b2b20218538