DevTools: show extension names instead of their ids in navigator view.

The patch adds ExtensionsSupport object which sits aside, waits for
execution contexts (which store extension names) to arrive and renames
matching contentscript projects.

BUG=160977
R=pfeldman

Review URL: https://codereview.chromium.org/599513002

git-svn-id: svn://svn.chromium.org/blink/trunk@182720 bbb929c8-8fbe-4397-9dbb-9b2b20218538