[GCM] Use the correct content type to talk with server for all users

BUG=384041

Review URL: https://codereview.chromium.org/561453002

Cr-Commit-Position: refs/heads/master@{#294984}

components/gcm_driver/gcm_channel_status_request.cc

@@ -21,7 +21,7 @@ namespace {
 
 const char kGCMChannelStatusRequestURL[] =
     "https://clients4.google.com/chrome-sync/command/";
-const char kRequestContentType[] = "application/x-protobuf";
+const char kRequestContentType[] = "application/octet-stream";
 const char kGCMChannelTag[] = "gcm_channel";
 const int kDefaultPollIntervalSeconds = 60 * 60;  // 60 minutes.
 const int kMinPollIntervalSeconds = 30 * 60;  // 30 minutes.