Remove unused WorkerGlobalScope::m_workerObservers HashSet

This was missed in https://src.chromium.org/viewvc/blink?view=rev&revision=167599,
which removed the WorkerGlobalScope::Observer interface.

R=tkent@chromium.org

Review URL: https://codereview.chromium.org/181663004

git-svn-id: svn://svn.chromium.org/blink/trunk@167953 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Request rebaseline for fast/text/chromium-linux-fontconfig-renderstyle.html.

BUG=342077
TEST=none
TBR=derat

Review URL: https://codereview.chromium.org/182563002

git-svn-id: svn://svn.chromium.org/blink/trunk@167954 bbb929c8-8fbe-4397-9dbb-9b2b20218538