tools/android/loading Loading trace database improvements

tools/android/loading/cloud/backend/README.md

 # Clovis in the Cloud: Developer Guide
 
 This document describes how to collect Chromium traces using Google Compute
 Engine.
 
 [TOC]
 
 ## Initial setup
 
 Install the [gcloud command line tool][1].
@@ skipping 77 matching lines @@
 ```
 
 The first time, you may need to get more access tokens:
 
 ```shell
 gcloud beta auth application-default login --scopes \
     https://www.googleapis.com/auth/cloud-taskqueue \
     https://www.googleapis.com/auth/cloud-platform
 ```
 
-Create a JSON file describing the deployment configuration:
+Create a JSON dictionary file describing the deployment configuration, with the
+keys:
 
+-   `project_name` (string): Name of the Google Cloud project
+-   `cloud_storage_path` (string): Path in Google Storage where generated traces
+    will be stored.
+-   `chrome_path` (string): Path to the Chrome executable.
+-   `src_path` (string): Path to the Chromium source directory.
+-   `taskqueue_tag` (string):
+-   `trace_database_filename` (string, optional): Filename for the trace
+    database in Cloud Storage. Must be unique per worker to avoid concurrent
+    access. Defaults to `trace_database.json`.
+
+Example:
 ```shell
-# CONFIG_FILE is the output json file.
-# PROJECT_NAME is the Google Cloud project.
-# CLOUD_STORAGE_PATH is the path in Google Storage where generated traces will
-# be stored.
-# CHROME_PATH is the path to the Chrome executable on the host.
-# CHROMIUM_SRC is the Chromium src directory.
 cat >$CONFIG_FILE << EOF
 {
   "project_name" : "$PROJECT_NAME",
   "cloud_storage_path" : "$CLOUD_STORAGE_PATH",
   "chrome_path" : "$CHROME_PATH",
   "src_path" : "$CHROMIUM_SRC",
   "taskqueue_tag" : "some_tag"
 }
 EOF
 ```
 
 Launch the app, passing the path to the deployment configuration file:
 
 ```shell
 python $CHROMIUM_SRC/tools/android/loading/cloud/backend/worker.py \
     --config $CONFIG_FILE
 ```
 
 You can now [use the app][2].
 
 Tear down the local environment:
 
 ```shell
 deactivate
 ```
 
 [1]: https://cloud.google.com/sdk
 [2]: #Use-the-app
 [3]: ../frontend/README.md