DevTools: Extract Toolbox.js into "toolbox_bootstrap" module

Drive-by: Fix devtools in the GN build

R=lushnikov, pfeldman, sergeyv

Review URL: https://codereview.chromium.org/594833002

git-svn-id: svn://svn.chromium.org/blink/trunk@182504 bbb929c8-8fbe-4397-9dbb-9b2b20218538