[WebSocket] Remove manual GC instructions from unittests.

Previously we need manual garbage collection instructions on WebSocket
unittests to suppress leak warnings. In addition to that, we cleared
references manually before the GC instruction.Since [1] does the garbage
collection automatically, we no longer have to do it.

1: https://src.chromium.org/viewvc/blink?view=rev&revision=179210
BUG=None

Review URL: https://codereview.chromium.org/622473002

git-svn-id: svn://svn.chromium.org/blink/trunk@183307 bbb929c8-8fbe-4397-9dbb-9b2b20218538