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().
This is changing the return value. Previously, the code would return a hash of {ok: false}, now it returns false. The same for the return true at the bottom, it used to return {ok: true, subtraces: subtraces}.