Support code coverage in javascript test

[Louis-Ye]

After fixing #791 , we should make the tests support code coverage to make the tests more useful.

If the web test is run through chromedp, then one option is to make use of the chrome devtools profiler (profiler.startPreciseCoverage method) to collect the coverage info of the javascript files, and convert it into the code coverage format (example format from the current report coverage.txt, generated by go -coverprofile in test.sh: github.com/google/pprof/profile/prune.go:152.2,152.12 1 2)

[aalexand]

Changed to type:cleanup since it's not really a feature - I suggest we use feature type only for user-visible feature requests.