shill: add some unit tests for ppp dongle support

This is the first in a series of CLs that will provide unit test coverage
for code introduced or modified by CL:51356, which added support for PPP
dongles.

The primary purpose of this CL is to add unit test coverage for the changes
made to cellular.cc. (Other files will be covered in other CLs.)

While there:
- add a PPPDeviceFactory, and a MockPPPDeviceFactory, so that we
  can force the creation of a Mock in Cellular::StartPPP
- add an mm1::MockModemSimpleProxy to CellularTest, so that the PPP
  tests can use something similar to what we use at runtime
- mock out SelectService and UpdateIPConfigFromPPP in MockPPPDevice,
  so that PPP tests can EXPECT on them
- update L2TPIPSecDriverTest.Notify to reflect that SelectService
  and UpdateIPConfigFromPPP are now mock methods
- add some comments in various proxy class headers, to help distinguish
  between stuff related to old modemmanager and stuff related to new
  modemmanager
- fix a random typo or two

BUG=chromium:246826
TEST=new unit tests

Change-Id: I5b2d0af0f9beb760d05e802531c0c9bc1ac5a040
Reviewed-on: https://gerrit.chromium.org/gerrit/63937
Tested-by: mukesh agrawal <quiche@chromium.org>
Reviewed-by: Thieu Le <thieule@chromium.org>
Commit-Queue: mukesh agrawal <quiche@chromium.org>
diff --git a/ppp_device_unittest.cc b/ppp_device_unittest.cc
index a73fa62..bf3b970 100644
--- a/ppp_device_unittest.cc
+++ b/ppp_device_unittest.cc
@@ -16,6 +16,8 @@
 
 namespace shill {
 
+// TODO(quiche): Add test for UpdateIPConfigFromPPP. crbug.com/266404
+
 TEST(PPPDeviceTest, GetInterfaceName) {
   map<string, string> config;
   config[kPPPInterfaceName] = "ppp0";