More flexible default google creds.

- We now accept the GOOGLE_APPLICATION_CREDENTIALS to point to a refresh
  token as opposed to just a json key.
- Tested with grpc_print_google_default_creds_token.
diff --git a/src/core/security/credentials.c b/src/core/security/credentials.c
index e79e9ce..10efc61 100644
--- a/src/core/security/credentials.c
+++ b/src/core/security/credentials.c
@@ -41,7 +41,6 @@
 #include "src/core/json/json.h"
 #include "src/core/httpcli/httpcli.h"
 #include "src/core/iomgr/iomgr.h"
-#include "src/core/security/json_token.h"
 #include "src/core/support/string.h"
 
 #include <grpc/support/alloc.h>
@@ -424,10 +423,9 @@
     jwt_destroy, jwt_has_request_metadata, jwt_has_request_metadata_only,
     jwt_get_request_metadata, NULL};
 
-grpc_credentials *grpc_jwt_credentials_create(const char *json_key,
-                                              gpr_timespec token_lifetime) {
+grpc_credentials *grpc_jwt_credentials_create_from_auth_json_key(
+    grpc_auth_json_key key, gpr_timespec token_lifetime) {
   grpc_jwt_credentials *c;
-  grpc_auth_json_key key = grpc_auth_json_key_create_from_string(json_key);
   if (!grpc_auth_json_key_is_valid(&key)) {
     gpr_log(GPR_ERROR, "Invalid input for jwt credentials creation");
     return NULL;
@@ -444,6 +442,12 @@
   return &c->base;
 }
 
+grpc_credentials *grpc_jwt_credentials_create(const char *json_key,
+                                              gpr_timespec token_lifetime) {
+  return grpc_jwt_credentials_create_from_auth_json_key(
+      grpc_auth_json_key_create_from_string(json_key), token_lifetime);
+}
+
 /* -- Oauth2TokenFetcher credentials -- */
 
 /* This object is a base for credentials that need to acquire an oauth2 token
@@ -787,12 +791,9 @@
   gpr_free(body);
 }
 
-grpc_credentials *grpc_refresh_token_credentials_create(
-    const char *json_refresh_token) {
+grpc_credentials *grpc_refresh_token_credentials_create_from_auth_refresh_token(
+    grpc_auth_refresh_token refresh_token) {
   grpc_refresh_token_credentials *c;
-  grpc_auth_refresh_token refresh_token =
-      grpc_auth_refresh_token_create_from_string(json_refresh_token);
-
   if (!grpc_auth_refresh_token_is_valid(&refresh_token)) {
     gpr_log(GPR_ERROR, "Invalid input for refresh token credentials creation");
     return NULL;
@@ -805,6 +806,12 @@
   return &c->base.base;
 }
 
+grpc_credentials *grpc_refresh_token_credentials_create(
+    const char *json_refresh_token) {
+  return grpc_refresh_token_credentials_create_from_auth_refresh_token(
+      grpc_auth_refresh_token_create_from_string(json_refresh_token));
+}
+
 /* -- Fake Oauth2 credentials. -- */
 
 typedef struct {