google: use Credentials instead of deprecated DefaultCredentials

Change-Id: Ie2d972df025f6b6b7d172015aae7eb34e355cb28
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/412034
Reviewed-by: Matt Hickford <matt.hickford@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cody Oss <codyoss@google.com>
Reviewed-by: Matthew Hickford <hickford@google.com>
Run-TryBot: Matt Hickford <matt.hickford@gmail.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cody Oss <codyoss@google.com>

google/default.go

@@ -137,7 +137,7 @@ func FindDefaultCredentialsWithParams(ctx context.Context, params CredentialsPar
 	// use those credentials. App Engine standard second generation runtimes (>= Go 1.11)
 	// and App Engine flexible use ComputeTokenSource and the metadata server.
 	if appengineTokenFunc != nil {
-		return &DefaultCredentials{
+		return &Credentials{
 			ProjectID:   appengineAppIDFunc(ctx),
 			TokenSource: AppEngineTokenSource(ctx, params.Scopes...),
 		}, nil
@@ -147,7 +147,7 @@ func FindDefaultCredentialsWithParams(ctx context.Context, params CredentialsPar
 	// or App Engine flexible, use the metadata server.
 	if metadata.OnGCE() {
 		id, _ := metadata.ProjectID()
-		return &DefaultCredentials{
+		return &Credentials{
 			ProjectID:   id,
 			TokenSource: ComputeTokenSource("", params.Scopes...),
 		}, nil
@@ -194,7 +194,7 @@ func CredentialsFromJSONWithParams(ctx context.Context, jsonData []byte, params
 		return nil, err
 	}
 	ts = newErrWrappingTokenSource(ts)
-	return &DefaultCredentials{
+	return &Credentials{
 		ProjectID:   f.ProjectID,
 		TokenSource: ts,
 		JSON:        jsonData,
@@ -216,7 +216,7 @@ func wellKnownFile() string {
 	return filepath.Join(guessUnixHomeDir(), ".config", "gcloud", f)
 }
 
-func readCredentialsFile(ctx context.Context, filename string, params CredentialsParams) (*DefaultCredentials, error) {
+func readCredentialsFile(ctx context.Context, filename string, params CredentialsParams) (*Credentials, error) {
 	b, err := ioutil.ReadFile(filename)
 	if err != nil {
 		return nil, err