Inspector: turn WebInspector.inspectedPageDomain into a function

For consistency, turn the getter into a function.

NOTRY=true

Review URL: https://codereview.chromium.org/178783003

git-svn-id: svn://svn.chromium.org/blink/trunk@168125 bbb929c8-8fbe-4397-9dbb-9b2b20218538