wpt: Add documentation for device API testing methodology

Adds references to MojoJS to the README.md files for the Web Bluetooth,
WebHID, Web Serial and WebUSB APIs.

Also adds README files linking from the Chromium-specific WPT test
directories to the main documentation in WPT.

Bug: 983364
Change-Id: I8db37a0fad4b93a13b7c56d56fbefc400f7b7513
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3171406
Reviewed-by: Chris Mumford <cmumford@google.com>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#923995}

bluetooth/README.md

@@ -76,7 +76,7 @@ More details can be found in `generate.py` and `generate_test.py`.
 
 ## Chromium Implementation
 The Chromium implementation is provided by
-`../resources/chromium/web-bluetooth-test.js`.
+`../resources/chromium/web-bluetooth-test.js` using [MojoJS].
 
 The Chromium implementation is not included in stable Chrome builds since it
 would add too much to the binary size. On Chromium infrastructure, it is run
@@ -93,6 +93,7 @@ infrastructure.
 
 For more implementation details, see the [Web Bluetooth Service README].
 
+[MojoJS]: https://chromium.googlesource.com/chromium/src/+/refs/heads/main/docs/testing/web_platform_tests.md#mojojs
 [Web Bluetooth Service README]:
 https://chromium.googlesource.com/chromium/src.git/+/master/content/browser/bluetooth/README.md
 

serial/README.md

@@ -0,0 +1,20 @@
+# Web Serial Testing
+
+Automated testing for the [Web Serial API] relies on a test-only interface which
+must be provided by browsers under test. This is similar to [WebUSB] however
+there is no separate specification of the API other than the tests themselves
+and the Chromium implementation.
+
+Tests in this suite include `resources/automation.js` to detect and load the
+test API as needed.
+
+The Chromium implementation is provided by
+`../resources/chromium/fake-serial.js` using [MojoJS].
+
+Tests with the "-manual" suffix do not use the test-only interface and expect a
+real hardware device to be connected. The specific characteristics of the device
+are described in each test.
+
+[MojoJS]: https://chromium.googlesource.com/chromium/src/+/refs/heads/main/docs/testing/web_platform_tests.md#mojojs
+[WebUSB]: ../webusb
+[Web Serial API]: https://wicg.github.io/serial

webusb/README.md

@@ -7,6 +7,11 @@ In this test suite `resources/usb-helpers.js` detects and triggers
 the API to be loaded as needed.
 
 The Chromium implementation is provided by
-`../resources/chromium/webusb-test.js`.
+`../resources/chromium/webusb-test.js` using [MojoJS].
 
+Tests with the "-manual" suffix do not use the test-only interface and expect a
+real hardware device to be connected. The specific characteristics of the device
+are described in each test.
+
+[MojoJS]: https://chromium.googlesource.com/chromium/src/+/refs/heads/main/docs/testing/web_platform_tests.md#mojojs
 [WebUSB Testing API]: https://wicg.github.io/webusb/test/