G-services settings v3 implementation

G-services settings v3 implementation:
* Adding calculation of settings based on a diff from server
* Adding calculation of settings digest
* Changing internal representation of settings to string->string map
  in order to be able to calculate the digest properly.
* Individual settings are calculated when needed.

R=jianli@chromium.org,zea@chromium.org
BUG=359256

Review URL: https://codereview.chromium.org/288433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271600 0039d316-1c4b-4281-b951-d872f2087c98

google_apis/gcm/engine/checkin_request.cc

@@ -17,7 +17,7 @@ namespace gcm {
 
 namespace {
 const char kRequestContentType[] = "application/x-protobuf";
-const int kRequestVersionValue = 2;
+const int kRequestVersionValue = 3;
 const int kDefaultUserSerialNumber = 0;
 
 // This enum is also used in an UMA histogram (GCMCheckinRequestStatus