Change SimulatedMouseEvent's adoptRef() to adoptRefWillBeRefCountedGarbageCollected()

I forgot to replace a couple of adoptRef()s in r170164.
SimulatedMouseEvent's adoptRef() was another left-over.

BUG=340522
TBR=ager

Review URL: https://codereview.chromium.org/216283002

git-svn-id: svn://svn.chromium.org/blink/trunk@170266 bbb929c8-8fbe-4397-9dbb-9b2b20218538