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().
Sorry, but I am not able to get your concern with this CL.

Why would importer working change this CL? Trace2HTMLImporter is returning exactly same return values as we do currently. Even with this CL it will work exactly as it does currently and will throw same errors as being done currently.

No functionality is getting changed with this refactoring CL. As for your concern, this is just small refactoring which neither changes design/functionality nor return values. That is the reason splitting the CL into 2 doesn't make any sense.

I am sorry if any of my comments gave you that impression.