[ChromeDriver] Add info on WPT WebDriver tests

NOTRY=True
TBR=wenbinzhang@google.com

Change-Id: Ib644820981abd902cb6372d3008c1ee0b372b9d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288100Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787629}

chrome/test/chromedriver/docs/testing.md

@@ -126,7 +126,59 @@ The Web Platform Tests (WPT) project is a W3C-coordinated attempt to build a
 cross-browser testsuit to verify how well browsers conform to web platform
 standards. Here, we will only focus on the WebDriver portion of WPT.
 
-TODO: Add details.
+To run WPT WebDriver tests, first clone the tests from GitHub into an empty
+directory:
+
+```
+git clone https://github.com/web-platform-tests/wpt
+```
+
+If necessary, install Python `virtualenv` module on your system.
+This only needs to be done once. The command for the installation depends on
+your system, but is usually something like:
+
+```
+pip install virtualenv
+```
+
+Now you can change into the WPT repository location,
+and run WPT WebDriver tests with the following command:
+
+```
+./wpt run [options] chrome webdriver
+```
+
+Use `./wpt run --help` to see all available options.
+The following are the most useful options:
+
+* `--webdriver-binary /path/to/chromedriver` specifies the ChromeDriver binary
+  to use. Without this option, the test runner will try to find ChromeDriver on
+  your PATH, or download ChromeDriver if it is not already on the PATH.
+* `--binary /path/to/chrome` specifies the Chrome binary to use.
+* `--webdriver-arg=...` specifies additional arguments to be passed to the
+  ChromeDriver command line. For example, to create a ChromeDriver verbose log,
+  use `--webdriver-arg=--verbose --webdriver-arg=--log-path=/path/to/log
+  --webdriver-arg=--append-log`. Note the following:
+  * Each ChromeDriver switch needs a separate `--webdriver-arg`.
+    Don't concatenate multiple switches together.
+  * Each ChromeDriver switch must be connected with `--webdriver-arg` with
+    an `=` sign.
+  * `--webdriver-arg=--append-log` is recommended.
+    Sometimes the test runner needs to restart ChromeDriver during tests,
+    and this can cause ChromeDriver to overwrite logs without this switch.
+* `--log-wptreport /path/to/report` generates a test report in JSON format.
+  The test runner always displays test failures on the screen, but it is often
+  useful to generate a test log as well. Many log formats are available,
+  see WPT help for all options.
+
+The WPT WebDriver tests are organized into subdirectories, one for each
+WebDriver command defined in the W3C spec. You can select a subset of the tests
+with the last argument on the WPT command line. For example, to run all tests
+for the New Session command, use
+
+```
+./wpt run [options] chrome webdriver/tests/new_session
+```
 
 ## JavaScript Unit Tests