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().
Nope. Currently also nowhere we use the return value as object. In fact, we cannot use the object format outside Trace2HTMLImporter since no other Importer class returns in object format. Hence, the return values for canImport() and extractSubtraces will always be boolean and Array of binary data only.

Current usage for both the functions is: TraceModel.importerConstructors_[i].canImport(eventData) and
var subtraces = importers[i].extractSubtraces();

So, current change is perfectly safe and compatible with users calling these API.