Drop [LegacyImplementedInBaseClass] from AbstractWorker IDL interface

Drop [LegacyImplementedInBaseClass] from AbstractWorker IDL interface.

R=haraken
BUG=344224
NOTRY=true

Review URL: https://codereview.chromium.org/168423003

git-svn-id: svn://svn.chromium.org/blink/trunk@167263 bbb929c8-8fbe-4397-9dbb-9b2b20218538