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().
So, how does the importer work now? Does it just fail to import the traces?

My preference is to split this into two CLs. One to do the cleanup refactoring (this CL) and one to change the return values to match other importers. Keeps things smaller and keeps the CLs doing a specific change.