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().
Yes, I am changing the return value of the function _extractEventsFromHTML function.

However, as I understand, _extractEventsFromHTML() is a private function of Trace2HTMLImporter class and should not be accessible outside this class i.e. atleast as per our JS conventions, it shouldn't be referred outside Trace2HTMLImporter class since it is a private member of this class.

So, it shouldn't matter if we change the return value of _extractEventsFromHTML(), as long as we are not changing the return values of the public functions Trace2HTMLImporter.canImport() and Trace2HTMLImporter.extractSubtraces(), which is not happening in this CL.

The newly added function of _canImportFromHTML() is also a private function and will not be accessible outside of Trace2HTMLImporter class. So, return type of that function also shouldn't matter outside the class.

To further ensure that I am not breaking anything, I have debugged by checking typeof return value and can confirm that the return type of Trace2HTMLImporter.canImport() and Trace2HTMLImporter.extractSubtraces() before and after my change is exactly same. 

The debugging output as I extracted from the console before my change:
typeof(_extractEventsFromHTML(events, false).ok)
"boolean"
typeof(_extractEventsFromHTML(events, false).subtraces)
"undefined"
_extractEventsFromHTML(events, true).ok
true
_extractEventsFromHTML(events, true).subtraces
[ArrayBuffer, ArrayBuffer]

The debugging output as I extracted from the console after my change:
typeof(_canImportFromHTML(events))
"boolean"
typeof(Trace2HTMLImporter.subtraces_)
"object"
Trace2HTMLImporter.subtraces_
[ArrayBuffer, ArrayBuffer]

Please correct me if I am still missing something important here.