Version: current ToT.

With block coverage enabled in V8, it's now possible for functions to have mixed-granularity coverage in DevTools. This may be problematic if we initially collect block-granularity coverage, and later switch to function-granularity - in this case, block-coverage is still displayed but never updated.

See attached screen capture for more details. 

1. The sequence from 0:36 to 1:00 shows the problematic workflow that happens when first collecting *with reload*, stopping collection, and then collecting *without reload*.
2. The source display seems to be bugged and does not display coverage information when restarting coverage collection at 0:32. It only works after restarting collection a second time. I've seen this a couple of times.

Full protocol:

0:02: Start instrumenting coverage and reload
      Note how the ranges are displayed in the source pane.
0:25: Clear all
0:32: Start instrumenting coverage and reload
      Note that the source pane seems to be bugged and does not display any ranges.
0:36: Clear all, Start instrumenting coverage and reload
      This time it works.
      "Hello" and "Bye" buttons clicked, compiling both functions and collecting partial coverage.
0:46: Clear all
0:49: Start instrumenting coverage *without reload*
      "Goodbye" and "Hi" buttons are clicked. Expected outcome: coverage should be updated.
      Actual outcome: coverage is not updated
      because we are now at function-granularity (the previous click on 'Stop collection' has deleted
      all information needed to collect block-granularity coverage).

One possible solution that I could imagine is removing the 'Clear all' button and making it implicit at the beginning of both 'Start instrumenting' options. The downside is that we'd lose any previously recorded best-effort coverage.
 

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