Make Elliott Sprehn a Source/web owner

Elliott has 25 patches in Source/web and has been doing reviews in this
area for some time. This patch makes his OWNER status official.

NOTRY=true

Review URL: https://codereview.chromium.org/293323002

git-svn-id: svn://svn.chromium.org/blink/trunk@174743 bbb929c8-8fbe-4397-9dbb-9b2b20218538