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DISALLOW_IMPLICIT_CONSTRUCTORS() should be used when we don't want any
implicit functions; but in the cases this CL touches, we just want the
default constructor to be private, not disallowed.

BUG=375000

Review URL: https://codereview.chromium.org/291063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271592 0039d316-1c4b-4281-b951-d872f2087c98