shill: cellular: Add CellularBearer class.

This CL adds a CellularBearer class to observe property changes from
bearer object exposed by ModemManager.

BUG=chromium:233918
TEST=Build and run unit tests.

Change-Id: I569fe21bf0059c60d1cb80979be59a8d906f1e30
Reviewed-on: https://chromium-review.googlesource.com/184761
Reviewed-by: Thieu Le <thieule@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/shill.gyp b/shill.gyp
index 54bc098..1706e48 100644
--- a/shill.gyp
+++ b/shill.gyp
@@ -183,6 +183,7 @@
           'sources': [
             'active_passive_out_of_credits_detector.cc',
             'cellular.cc',
+            'cellular_bearer.cc',
             'cellular_capability.cc',
             'cellular_capability_cdma.cc',
             'cellular_capability_classic.cc',
@@ -658,6 +659,7 @@
             ['USE_cellular == 1', {
               'sources': [
                 'active_passive_out_of_credits_detector_unittest.cc',
+                'cellular_bearer_unittest.cc',
                 'cellular_capability_cdma_unittest.cc',
                 'cellular_capability_classic_unittest.cc',
                 'cellular_capability_gsm_unittest.cc',