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().
No we are actually returning the same return value even in current implementation.

Earlier _extractEventsFromHTML(events, false) was returning an object with 2 keys: {ok and subtraces}, with both keys values being set. The values of key 'ok' used to be true or false, depending on whether import is success or not. The value of key 'subtraces' used to be array of binary data or undefined. 

In Trace2HTMLImporter.canImport, we were returning _extractEventsFromHTML(events, false).ok;
implies we were returning the value of key 'ok' in the object returned from _extractEventsFromHTML which used to be true or false.

Similarly, in Trace2HTMLImporter.extractSubtraces we were returning _extractEventsFromHTML(events, false).subtraces;
implies we were returning the value of key 'subtraces' in the object returned from _extractEventsFromHTML which used to be array of binary data.

In current implementation, I am returning the same values directly, without use of the object.
Correct me if you think otherwise.