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Nothing sets it to false anymore. This also means we can
remove all the machinery around setting this via
enterForceCompositingMode. Unfortunately, WebPopupMenuImpl
still needs enterForceCompositingMode. That's crbug.com/378029.

BUG=363772,378029

Review URL: https://codereview.chromium.org/295383008

git-svn-id: svn://svn.chromium.org/blink/trunk@175027 bbb929c8-8fbe-4397-9dbb-9b2b20218538