Add explicit to one-argument constructors

I was looking at this code while profiling a site and noticed that we were
missing some explicit keywords.

R=eseidel

Review URL: https://chromiumcodereview.appspot.com/22474004

git-svn-id: svn://svn.chromium.org/blink/trunk@155656 bbb929c8-8fbe-4397-9dbb-9b2b20218538