[Android WebAPK] Make webapk_installer.cc return proto as base64 string

chrome/browser/android/webapk/webapk_install_service.cc

Index: chrome/browser/android/webapk/webapk_install_service.cc
diff --git a/chrome/browser/android/webapk/webapk_install_service.cc b/chrome/browser/android/webapk/webapk_install_service.cc
index 22dcbbeae941236cd6b8c7688427a80f1e0b2501..12d8952b118284c5602a7fd37516a999dc3f7e9d 100644
--- a/chrome/browser/android/webapk/webapk_install_service.cc
+++ b/chrome/browser/android/webapk/webapk_install_service.cc
@@ -45,7 +45,7 @@ void WebApkInstallService::UpdateAsync(
     const std::string& webapk_package,
     const GURL& start_url,
     const base::string16& short_name,
-    std::unique_ptr<std::vector<uint8_t>> serialized_proto,
+    std::unique_ptr<std::string> serialized_proto,
     const FinishCallback& finish_callback) {
   WebApkInstaller::UpdateAsync(browser_context_, webapk_package, start_url,
                                short_name, std::move(serialized_proto),