Simplify blink::createSameThreadTask using variadic templates.

SSIA. This improves the code readability and decreases its size.

Review URL: https://codereview.chromium.org/622503002

git-svn-id: svn://svn.chromium.org/blink/trunk@183129 bbb929c8-8fbe-4397-9dbb-9b2b20218538