DevTools: New application loader framework implementation

This patch splits the frontend into 2 disparate applications: 
- devtools (the primary application),
- toolbox (the device emulation frontend for detached devtools window).

The design doc is referenced from a tracking issue comment.

BUG=391566

Review URL: https://codereview.chromium.org/483783002

git-svn-id: svn://svn.chromium.org/blink/trunk@182595 bbb929c8-8fbe-4397-9dbb-9b2b20218538