DevTools: Optimize scriptParsed event handling in front-end.

Replaced some object property based maps with native Maps.
Optimized console.assert.
Optimized CompilerScriptMapping for scripts without source map.
Optimized some loops over iterators.
Extracted try-catch block to a separate method in ParsedURL.

R=pfeldman, apavlov

Review URL: https://codereview.chromium.org/684013003

git-svn-id: svn://svn.chromium.org/blink/trunk@184704 bbb929c8-8fbe-4397-9dbb-9b2b20218538