Avoid multiple calls to _extractTracesFromHTML in Trace2HTMLImporter.

For each HTML import, we trigger multiple calls to _extractTracesFromHTML.
In Trace2HTMLImporter.canImport() and in Importer.extractSubtraces().
We can instead cache the value of subtraces also during canImport and
return the cached value during extractSubtraces().
Right, you're returning the same value directly, but it's still a different return value. What do clients of this API expect? Currently they're probably all doing something like:

```
var val = importer.extractSubtraces();
if (val.ok) {
  ...
}
```

which, won't work anymore as the return value isn't a hash any longer.