Add *explicit* keyword to avoid implicit type conversion

Constructors of RemoteFontFaceSource, SVGDocumentExtensions, XMLDocumentParserScope, and WorkerScriptDebugServer need to
use *explicit* keyword because they have an argument.

BUG=N/A

Review URL: https://codereview.chromium.org/207103003

git-svn-id: svn://svn.chromium.org/blink/trunk@169709 bbb929c8-8fbe-4397-9dbb-9b2b20218538