Fix cast from video player by using DriveApiUrlGenerator to generate URL

This patch fixes the regression which is caused by the following CL.
https://chromium.googlesource.com/chromium/src/+/3315eb5fe7bcb9300995dae3326de2d2c5f3c44a

Since the Styx URLs are updated in that CL, we need to use DriveApiUrlGenerator::GenerateDownloadFileURL to create a download URL to pass a cast device.

BUG=477733
TEST=can cast a video

Review URL: https://codereview.chromium.org/1055403004

Cr-Commit-Position: refs/heads/master@{#326999}

chrome/browser/chromeos/extensions/file_manager/private_api_drive.cc

@@ -1034,9 +1034,10 @@ void FileManagerPrivateGetDownloadUrlFunction::OnGetResourceEntry(
     return;
   }
 
-  download_url_ =
-      google_apis::DriveApiUrlGenerator::kBaseDownloadUrlForProduction +
-      entry->resource_id();
+  DriveApiUrlGenerator url_generator(
+      (GURL(google_apis::DriveApiUrlGenerator::kBaseUrlForProduction)),
+      (GURL(google_apis::DriveApiUrlGenerator::kBaseDownloadUrlForProduction)));
+  download_url_ = url_generator.GenerateDownloadFileUrl(entry->resource_id());
 
   ProfileOAuth2TokenService* oauth2_token_service =
       ProfileOAuth2TokenServiceFactory::GetForProfile(GetProfile());
@@ -1065,7 +1066,8 @@ void FileManagerPrivateGetDownloadUrlFunction::OnTokenFetched(
     return;
   }
 
-  const std::string url = download_url_ + "?access_token=" + access_token;
+  const std::string url =
+      download_url_.Resolve("?access_token=" + access_token).spec();
   SetResult(new base::StringValue(url));
 
   SendResponse(true);

chrome/browser/chromeos/extensions/file_manager/private_api_drive.h

@@ -240,7 +240,7 @@ class FileManagerPrivateGetDownloadUrlFunction
                       const std::string& access_token);
 
  private:
-  std::string download_url_;
+  GURL download_url_;
   scoped_ptr<google_apis::AuthService> auth_service_;
 };