Use fixed-sized types

Should have caught this during review, but cpplint caught it

Author: wilhuff

Firestore/core/src/firebase/firestore/model/timestamp.cc

@@ -24,7 +24,7 @@ namespace firebase {
 namespace firestore {
 namespace model {
 
-Timestamp::Timestamp(long seconds, int nanos)
+Timestamp::Timestamp(int64_t seconds, int32_t nanos)
     : seconds_(seconds), nanos_(nanos) {
   FIREBASE_ASSERT_MESSAGE_WITH_EXPRESSION(
       nanos >= 0, nanos >= 0, "timestamp nanoseconds out of range: %d", nanos);

Firestore/core/src/firebase/firestore/model/timestamp.h

@@ -43,22 +43,22 @@ class Timestamp {
    * @param seconds the number of seconds since epoch.
    * @param nanos the number of nanoseconds after the seconds.
    */
-  Timestamp(long seconds, int nanos);
+  Timestamp(int64_t seconds, int32_t nanos);
 
   /** Returns a timestamp with the current date / time. */
   static Timestamp Now();
 
-  long seconds() const {
+  int64_t seconds() const {
     return seconds_;
   }
 
-  int nanos() const {
+  int32_t nanos() const {
     return nanos_;
   }
 
  private:
-  long seconds_;
-  int nanos_;
+  int64_t seconds_;
+  int32_t nanos_;
 };
 
 /** Compares against another Timestamp. */
@@ -91,4 +91,4 @@ inline bool operator==(const Timestamp& lhs, const Timestamp& rhs) {
 }  // namespace firestore
 }  // namespace firebase
 
-#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_FIELD_VALUE_H_
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_TIMESTAMP_H_