google: add CredentialsFromJSON

Support obtaining a DefaultCredentials value from JSON data.

Also, add an example, and write more package doc.

For Go 1.9 and higher, rename DefaultCredentials to
Credentials and make the former an alias for the latter.

Updates google/google-api-go-client#247.

Change-Id: I9f9e234ed79f8e08fa13914d9c6c60e0154a06e5
Reviewed-on: https://go-review.googlesource.com/99795
Reviewed-by: Ross Light <light@google.com>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

google/example_test.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build appenginevm appengine
-
 package google_test
 
 import (
@@ -12,6 +10,7 @@ import (
 	"log"
 	"net/http"
 
+	"golang.org/x/net/context"
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
 	"golang.org/x/oauth2/jwt"
@@ -148,3 +147,16 @@ func ExampleComputeTokenSource() {
 	}
 	client.Get("...")
 }
+
+func ExampleCredentialsFromJSON() {
+	ctx := context.Background()
+	data, err := ioutil.ReadFile("/path/to/key-file.json")
+	if err != nil {
+		log.Fatal(err)
+	}
+	creds, err := google.CredentialsFromJSON(ctx, data, "https://www.googleapis.com/auth/bigquery")
+	if err != nil {
+		log.Fatal(err)
+	}
+	_ = creds // TODO: Use creds.
+}