DevTools: Get rid of module initializers in the source tree

The _module.js files are currently used only at build-time in the
release mode and at run-time when starting workers.
We have all the required module info in module.json files,
and workers can be loaded in the debug mode through a blob containing
concatenated scripts for the worker, loaded via sync XHRs.
This patch also gets rid of common/modules.js: module descriptors
are now inlined into Runtime.js.

R=pfeldman, vsevik
BUG=391566

Review URL: https://codereview.chromium.org/472903003

git-svn-id: svn://svn.chromium.org/blink/trunk@180452 bbb929c8-8fbe-4397-9dbb-9b2b20218538