DevTools: Wait for resource tree model init before loading source maps.
DevTools: Wait for resource tree model init before loading source maps.

BUG=350666

Review URL: https://codereview.chromium.org/429893005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@179300 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs