Missing explicit keyword in ctor of FooImpl.h

Need to add "explicit" to ctor which only has one argument in order to
avoid implicit type conversion. Additionally some style nits are fixed together.

BUG=none

Review URL: https://codereview.chromium.org/423793004

git-svn-id: svn://svn.chromium.org/blink/trunk@179226 bbb929c8-8fbe-4397-9dbb-9b2b20218538