Missing *explicit* keyword in constructors

Constructor which has one parameter needs to have explicit keyword
in order to avoid implicit type conversion.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23531058

git-svn-id: svn://svn.chromium.org/blink/trunk@157807 bbb929c8-8fbe-4397-9dbb-9b2b20218538