ExtensionStorageMonitor: use smaller, self-registering StorageObservers

chrome/test/data/extensions/storage_monitor/hosted_apps/common.js

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Opens the filesystem and returns a Promise that resolves to the opened
+// filesystem.
+function GetFileSystem(type, size) {
+  return new Promise(
+      (resolve, reject) =>
+          webkitRequestFileSystem(type, size, resolve, reject));
+}
+
+// Returns a .then()-chainable handler that accepts a fileystem, creates a file,
+// and returns a Promise that resolves to the successfully created file.
+function CreateFile(filename) {
+  return (filesystem) => new Promise(
+             (resolve, reject) => filesystem.root.getFile(
+                 filename, {create: true}, resolve, reject));
+}
+
+// Returns a .then()-chainable handler that accepts a filesystem file, appends
+// |numChars| to it, and returns a Promise that resolves to the file once the
+// append operation is successful.
+function AppendDataToFile(numChars) {
+  return ((fileEntry) => {
+    return new Promise((resolve, reject) => {
+      fileEntry.createWriter((fileWriter) => {
+        // FileWriter's onwriteend resolves the promise; onerror rejects it.
+        fileWriter.onwriteend = (e) => resolve(fileEntry);
+        fileWriter.onerror = reject;
+        fileWriter.seek(fileWriter.length);
+
+        var str = 'a'.repeat(numChars);
+
+        var blob = new Blob([str], {type: 'text/plain'});
+        console.assert(blob.size == numChars);
+        fileWriter.write(blob);
+      }, reject);
+    });
+  });
+}
+
+// Entry point to be called via ExecuteScript in the browser test C++ code.
+//
+// Asynchronously opens writes |numChars| to the filesystem. Returns a Promise
+// that resolves upon completion.
+function HostedAppWriteData(type, numChars) {
+  return GetFileSystem(type, 16384)
+      .then(CreateFile('test.txt'))
+      .then(AppendDataToFile(numChars));
+}