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().
It does change the return values. It may line them up more with the current return values, but it still does change what this importer is returning.

The previous return values were hashes, they are now either an array or true/false.

I think the fix of lining up the return values with other importers should be in a separate CL instead of hidden within this CL. It will make it easier, if needed, to track things down later.