Update dependency google.golang.org/api to v0.15.1

Steps:
./hack/pin-dependency.sh google.golang.org/api v0.15.1
./hack/update-vendor.sh
./hack/verify-vendor.sh
./hack/pin-dependency.sh github.com/googleapis/gax-go/v2 v2.0.5
./hack/update-vendor.sh
./hack/verify-vendor.sh
./hack/lint-dependencies.sh

vendor/google.golang.org/api/googleapi/googleapi.go

@@ -1,4 +1,4 @@
-// Copyright 2011 Google Inc. All rights reserved.
+// Copyright 2011 Google LLC. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
@@ -16,7 +16,7 @@ import (
 	"net/url"
 	"strings"
 
-	"google.golang.org/api/googleapi/internal/uritemplates"
+	"google.golang.org/api/internal/third_party/uritemplates"
 )
 
 // ContentTyper is an interface for Readers which know (or would like
@@ -256,14 +256,22 @@ func ProcessMediaOptions(opts []MediaOption) *MediaOptions {
 // "http://www.golang.org/topics/myproject/mytopic". It strips all parent
 // references (e.g. ../..) as well as anything after the host
 // (e.g. /bar/gaz gets stripped out of foo.com/bar/gaz).
+//
+// ResolveRelative panics if either basestr or relstr is not able to be parsed.
 func ResolveRelative(basestr, relstr string) string {
-	u, _ := url.Parse(basestr)
+	u, err := url.Parse(basestr)
+	if err != nil {
+		panic(fmt.Sprintf("failed to parse %q", basestr))
+	}
 	afterColonPath := ""
 	if i := strings.IndexRune(relstr, ':'); i > 0 {
 		afterColonPath = relstr[i+1:]
 		relstr = relstr[:i]
 	}
-	rel, _ := url.Parse(relstr)
+	rel, err := url.Parse(relstr)
+	if err != nil {
+		panic(fmt.Sprintf("failed to parse %q", relstr))
+	}
 	u = u.ResolveReference(rel)
 	us := u.String()
 	if afterColonPath != "" {
@@ -331,7 +339,7 @@ func ConvertVariant(v map[string]interface{}, dst interface{}) bool {
 }
 
 // A Field names a field to be retrieved with a partial response.
-// See https://developers.google.com/gdata/docs/2.0/basics#PartialResponse
+// https://cloud.google.com/storage/docs/json_api/v1/how-tos/performance
 //
 // Partial responses can dramatically reduce the amount of data that must be sent to your application.
 // In order to request partial responses, you can specify the full list of fields
@@ -348,9 +356,6 @@ func ConvertVariant(v map[string]interface{}, dst interface{}) bool {
 //
 //     svc.Events.List().Fields("nextPageToken", "items(id,updated)").Do()
 //
-// More information about field formatting can be found here:
-// https://developers.google.com/+/api/#fields-syntax
-//
 // Another way to find field names is through the Google API explorer:
 // https://developers.google.com/apis-explorer/#p/
 type Field string