PasswordManagerHandler (PMH) has two methods, HandlePasswordImport and HandlePasswordExport, which are fairly independent of the rest or PMH.

This not only adds to the code bloat in PMH, but also makes its testing harder and more prone to mistakes (PMH now has its own testing subclass in its own unittest).

Instead we should separate the functionality of the two methods in a separate class (PasswordImportExportDialogHandler?), owned by PMH but unaware of PMH. The class could also remove some code duplication between the two methods. 

Furthermore, another new class (PasswordImportExportDialogListener?) should become the dialogue listener instead of PMH.

This will make unit testing through mocking much easier, and improve the code structure by decreasing the implicit dependencies and code bloat.

This is based on https://codereview.chromium.org/1193143003/diff/850001/chrome/browser/ui/webui/options/password_manager_handler_unittest.cc#newcode200.