Add Android-backend to the memory_inspector (prerequisites).

tools/memory_inspector/memory_inspector/backends/prebuilts_fetcher.py

Index: tools/memory_inspector/memory_inspector/backends/prebuilts_fetcher.py
diff --git a/tools/memory_inspector/memory_inspector/backends/prebuilts_fetcher.py b/tools/memory_inspector/memory_inspector/backends/prebuilts_fetcher.py
new file mode 100644
index 0000000000000000000000000000000000000000..703b508b8d79ffce3f5425494334b39b4f3c9b8d
--- /dev/null
+++ b/tools/memory_inspector/memory_inspector/backends/prebuilts_fetcher.py
@@ -0,0 +1,61 @@
+# Copyright 2014 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.
+
+"""This module fetches and syncs prebuilts from Google Cloud Storage (GCS).
+
+See prebuilts/README for a description of the respository <> GCS sync mechanism.
+"""
+
+import hashlib
+import logging
+import os
+import urllib
+
+
+_PREBUILTS_BUCKET = 'chromium-telemetry'
+
+# Bypass the GCS download logic in unittests and use the *_ForTests mock.
+in_test_harness = False
+
+
+def GetIfChanged(local_file_path):
+  """Downloads the file from GCS, only if the local one is outdated."""
+  is_changed = _IsChanged(local_file_path)
+
+  # In test harness mode we are only interested in checking that the proper
+  # .sha1 files have been checked in (hence the call to _IsChanged() above).
+  if in_test_harness:
+    return _GetIfChanged_ForTests(local_file_path)
+
+  if not is_changed:
+    return
+  obj_name = _GetRemoteFileID(local_file_path)
+  url = 'https://storage.googleapis.com/%s/%s' % (_PREBUILTS_BUCKET, obj_name)
+  logging.info('Downloading %s prebuilt from %s.' % (local_file_path, url))
+  urllib.urlretrieve(url, local_file_path)
+  assert(not _IsChanged(local_file_path)), 'GCS download failed.'
+
+
+def _IsChanged(local_file_path):
+  """Checks whether the local_file_path exists and matches the expected hash."""
+  is_changed = True
+  expected_hash = _GetRemoteFileID(local_file_path)
+  if os.path.exists(local_file_path):
+    with open(local_file_path, 'rb') as f:
+      local_hash = hashlib.sha1(f.read()).hexdigest()
+    is_changed = (local_hash != expected_hash)
+  return is_changed
+
+
+def _GetRemoteFileID(local_file_path):
+  """Returns the checked-in hash which identifies the name of file in GCS."""
+  hash_path = local_file_path + '.sha1'
+  with open(hash_path, 'rb') as f:
+    return f.read(1024).rstrip()
+
+
+def _GetIfChanged_ForTests(local_file_path):
+  """This is the mock version of |GetIfChanged| used only in unittests."""
+  # Just truncate / create an empty file.
+  open(local_file_path, 'wb').close()