[ChromeDriver] Add README.md to log_replay/

Added a readme to the chromedriver/log_replay/ directory with usage
and maintenance info for the log replay system.

Bug: chromedriver:2501
Change-Id: I0bef83149cd6be6dd0978d81f2f79600f96cde77
Reviewed-on: https://chromium-review.googlesource.com/1194573
Commit-Queue: Cole Winstanley <cwinstanley@google.com>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587273}

chrome/test/chromedriver/log_replay/README.md

+# ChromeDriver Log-Replay component
+This directory contains a component of ChromeDriver that allows developers to
+"replay" ChromeDriver sessions using a ChromeDriver log file.
+
+## Sample Usage
+For full usage information, run `./client_replay.py --help`.
+
+In general, this component is used by generating a ChromeDriver log from a
+session of interest (or getting the log from an bug report, etc.), and then
+replaying that log using this component. For example, to replay ChromeDriver
+tests from chromedriver/test/run_py_tests.py:
+```
+# Generate the log
+test/run_py_tests.py --log-path=<desired path to log> --replayable=true
+# Replay the log
+./client_replay.py <chromedriver binary> <chrome binary> --devtools-replay=true
+```
+For more usage information, run `./client_replay.py --help`.
+
+## Project Information
+Design: https://docs.google.com/a/google.com/document/d/e/2PACX-1vRsp_DhC797flAmzLFaX6Ly4Ro5I9wL-Xk_HH0Zibl6abQ3XTvIypZgdMcBn3hw1FhEoizOhK1RqJpA/pub
+
+## Architecture
+The code has two main components: the client-side replay written in Python and
+the DevTools-side replay written in C++.
+
+### Client-side Replay
+The client-side replay module is written in Python and exists separately from
+the ChromeDriver binary. It re-uses the same logic to launch the ChromeDriver
+binary as test/run_py_tests.py (this code is in the chromedriver/server
+directory).
+
+The entry point for the client-side replay is client_replay.py. This
+file contains code that parses client-side commands out of the log file and
+interprets them back into the corresponding WebDriver requests. When there are
+session, window, or element IDs in the log, the CommandSequence class
+substitutes them to match the new session so that the logged commands have
+valid references to sessions, windows, and elements.
+
+This entry point also launches ChromeDriver (which in turn launches Chrome),
+and has a programmatic interface for retrieving commands from a log file using
+the CommandSequence class. The command-line interface uses the Replayer class
+to interface with a CommandSequence.
+
+### DevTools-side Replay
+The DevTools-side replay module is written in C++ and is built as part of
+ChromeDriver binary, enabled by running ChromeDriver (or client_replay.py) with
+the `--devtools-replay=true` flag.
+
+DevToolsLogReader (in devtools_log_reader.h) parses out DevTools commands,
+responses, and events out of the log file. Depending on the DevTools command
+type (HTTP or WebSocket), the responses are read directly into ChromeDriver by
+either the ReplayHTTPClient or the LogReplaySocket classes.
+
+ReplayHTTPClient subclasses DevToolsHTTPClient, which is responsible for all of
+the HTTP communication between ChromeDriver and DevTools, and it overrides one
+method to communicate with the DevToolsLogReader instead of the actual Chrome
+instance. LogReplaySocket subclasses SyncWebSocket (see chromedriver/net/) and
+is used to redirect WebSocket communication from DevToolsClientImpl to the
+DevToolsLogReader instead of getting the WebSocket events and responses from
+Chrome.
+
+Between these two classes, all of the communication between ChromeDriver and
+Chrome is replicated, so when DevTools replay is enabled, no Chrome instance
+even needs to be launched. Note that the DevTools replay component can be run
+without running the client-side replay, but this doesn't make much sense as the
+same client-side commands would have to be called in the same order as in the
+logged session to replicate the same DevTools commands, and at that point you
+may as well replay the client side as well as the DevTools side.
+
+### Tests
+#### Unit tests
+The C++ unit tests for devtools_log_reader.cc are integrated into the
+chromedriver_unittests target. Python unit tests for the client-replay are
+standalone in the file client_replay_unittest.py but are integrated into the
+Chromium CQ.
+
+#### End-to-end tests
+client_replay_tests.py runs the end-to-end tests for both the client side and
+DevTools side (with the --devtools-replay=true flag) replay functions.
+
+## Maintenance
+There are a few places in this directory that need to be maintained based on
+external code. On changes in command names, the `_COMMANDS` list in
+client_replay.py will need to be updated (see
+https://crbug.com/chromedriver/2511 ).
+
+When there is significant changes to the format of logging (at spots marked with
+comments in the ChromeDriver code), the log reading will need to be updated
+accordingly, on both the DevTools side (devtools_log_reader.cc), and the client
+side (`_Parser` class in client_replay.py).
+
+If log formats change, it may be necessary to update the unit test data files
+(in test_data/) to match the new logging format. These are small sample log
+entries used only for unit tests; the end-to-end tests auto-generate the needed
+logs.
\ No newline at end of file