Cleanup early returns in SourcesPanel.js
Cleanup early returns in SourcesPanel.js

The big 'if-with-braces' CL made some early return statements less
readable. This CL fixes that by flipping the logic and removing early
returns, as well as some other minor cleanups in SourcesPanel.js.

R=17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org

Change-Id: Iecc8da4177a175d5a42413668f22497d3d7ad096
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1993346
Commit-Queue: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>
Reviewed-by: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>

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