Get rid of implicit conversions to VisibleSelection.

Some constructors of VisibleSelection accepting just one argument (with
optional arguments omitted) are not marked as explicit. This creates implicit
conversions from the constructor's argument type to VisibleSelection,
which is not desirable since it's very easy to make the conversion
inadvertently.

Code refactoring only, no behavior change.

BUG=none

Review URL: https://codereview.chromium.org/194263002

git-svn-id: svn://svn.chromium.org/blink/trunk@168895 bbb929c8-8fbe-4397-9dbb-9b2b20218538