Document cellular properties

Currently the Cellular properties are not documented in onc_spec.html
at all. This provides rudimentary documentation based on the Shill
documentaiton.

It also removes 'SelectedNetwork' which is undocumented in Shill and
unused in Chrome.

BUG=None

Review URL: https://codereview.chromium.org/563643002

Cr-Commit-Position: refs/heads/master@{#294516}

chromeos/network/onc/onc_signature.cc

@@ -275,7 +275,6 @@ const OncFieldSignature cellular_with_state_fields[] = {
     { ::onc::cellular::kPRLVersion, &kIntegerSignature},
     { ::onc::cellular::kProviderRequiresRoaming, &kBoolSignature},
     { ::onc::cellular::kRoamingState, &kStringSignature},
-    { ::onc::cellular::kSelectedNetwork, &kStringSignature},
     { ::onc::cellular::kServingOperator, &kCellularProviderSignature},
     { ::onc::cellular::kSIMLockStatus, &kSIMLockStatusSignature},
     { ::onc::cellular::kSIMPresent, &kBoolSignature},

chromeos/network/onc/onc_translation_tables.cc

@@ -338,7 +338,6 @@ const FieldTranslationEntry kCellularDeviceTable[] = {
     { ::onc::cellular::kPRLVersion, shill::kPRLVersionProperty},
     { ::onc::cellular::kProviderRequiresRoaming,
       shill::kProviderRequiresRoamingProperty},
-    { ::onc::cellular::kSelectedNetwork, shill::kSelectedNetworkProperty},
     // This field is converted during translation, see onc_translator_*.
     // { ::onc::cellular::kSIMLockStatus, shill::kSIMLockStatusProperty},
     { ::onc::cellular::kSIMPresent, shill::kSIMPresentProperty},

components/onc/onc_constants.cc

@@ -115,7 +115,6 @@ const char kProviderRequiresRoaming[] = "ProviderRequiresRoaming";
 const char kRoamingState[] = "RoamingState";
 const char kHome[] = "Home";
 const char kRoaming[] = "Roaming";
-const char kSelectedNetwork[] = "SelectedNetwork";
 const char kServingOperator[] = "ServingOperator";
 const char kSIMLockStatus[] = "SIMLockStatus";
 const char kSIMPresent[] = "SIMPresent";

components/onc/onc_constants.h

@@ -133,7 +133,6 @@ ONC_EXPORT extern const char kProviderRequiresRoaming[];
 ONC_EXPORT extern const char kRoamingState[];
 ONC_EXPORT extern const char kHome[];
 ONC_EXPORT extern const char kRoaming[];
-ONC_EXPORT extern const char kSelectedNetwork[];
 ONC_EXPORT extern const char kServingOperator[];
 ONC_EXPORT extern const char kSIMLockStatus[];
 ONC_EXPORT extern const char kSIMPresent[];