google: add DefaultCredentials function

This new function allows reading the project ID from a service account
JSON file without an additional disk read.

Change-Id: I1f03ca3ca39a2ae3bd6524367c17761b0f08de45
Reviewed-on: https://go-review.googlesource.com/32876
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>

google/google.go

@@ -112,6 +112,7 @@ type credentialsFile struct {
 	PrivateKeyID string `json:"private_key_id"`
 	PrivateKey   string `json:"private_key"`
 	TokenURL     string `json:"token_uri"`
+	ProjectID    string `json:"project_id"`
 
 	// User Credential fields
 	// (These typically come from gcloud auth.)