Use a valid ChromeOS #define in CheckinRequest

CheckinRequest uses a non-existent #define for ChromeOS,
defined(CHROME_OS), we should replace it with defined(OS_CHROMEOS).

Bug: 1121653
Change-Id: Ieae0bdf874c006296bfedc4da3cd2394b4e3a9ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434777Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811519}

google_apis/gcm/engine/checkin_request.cc

@@ -138,7 +138,7 @@ void CheckinRequest::Start() {
 
   checkin_proto::AndroidCheckinProto* checkin = request.mutable_checkin();
   checkin->mutable_chrome_build()->CopyFrom(request_info_.chrome_build_proto);
-#if defined(CHROME_OS)
+#if defined(OS_CHROMEOS)
   checkin->set_type(checkin_proto::DEVICE_CHROME_OS);
 #else
   checkin->set_type(checkin_proto::DEVICE_CHROME_BROWSER);

google_apis/gcm/engine/checkin_request_unittest.cc

@@ -153,7 +153,7 @@ TEST_F(CheckinRequestTest, FetcherDataAndURL) {
   EXPECT_EQ(kEmailAddress, request_proto.account_cookie(0));
   EXPECT_EQ(kTokenValue, request_proto.account_cookie(1));
 
-#if defined(CHROME_OS)
+#if defined(OS_CHROMEOS)
   EXPECT_EQ(checkin_proto::DEVICE_CHROME_OS, request_proto.checkin().type());
 #else
   EXPECT_EQ(checkin_proto::DEVICE_CHROME_BROWSER,