google: support AWS 3rd party credentials

Change-Id: I655b38f7fb8023866bb284c7ce80ab9888682e73
GitHub-Last-Rev: 648f0b3d45d94760bb29e6bfe4680351d8e0364d
GitHub-Pull-Request: golang/oauth2#471
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/287752
Reviewed-by: Cody Oss <codyoss@google.com>
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Tyler Bui-Palsulich <tbp@google.com>
Trust: Cody Oss <codyoss@google.com>

google/internal/externalaccount/filecredsource_test.go

@@ -56,7 +56,12 @@ func TestRetrieveFileSubjectToken(t *testing.T) {
 		tfc.CredentialSource = test.cs
 
 		t.Run(test.name, func(t *testing.T) {
-			out, err := tfc.parse(context.Background()).subjectToken()
+			base, err := tfc.parse(context.Background())
+			if err != nil {
+				t.Fatalf("parse() failed %v", err)
+			}
+
+			out, err := base.subjectToken()
 			if err != nil {
 				t.Errorf("Method subjectToken() errored.")
 			} else if test.want != out {