Fix saving files encoded in base64

A follow-up to https://crrev.com/c/1230042.

Change-Id: I7b786f41db42332c5addbd40b5a61a90e2764694
Reviewed-on: https://chromium-review.googlesource.com/c/1283750Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Ingvar Stepanyan <ingvar@cloudflare.com>
Cr-Commit-Position: refs/heads/master@{#600441}

third_party/blink/renderer/devtools/front_end/persistence/PersistenceActions.js

@@ -20,7 +20,9 @@ Persistence.PersistenceActions.ContextMenuProvider = class {
     async function saveAs() {
       if (contentProvider instanceof Workspace.UISourceCode)
         /** @type {!Workspace.UISourceCode} */ (contentProvider).commitWorkingCopy();
-      const content = await contentProvider.requestContent();
+      let content = await contentProvider.requestContent();
+      if (await contentProvider.contentEncoded())
+        content = window.atob(content);
       const url = contentProvider.contentURL();
       Workspace.fileManager.save(url, /** @type {string} */ (content), true);
       Workspace.fileManager.close(url);