bluetooth: Error for unsupported environment

This change adds a thrown error message for browser environments
that do not implement Web Bluetooth Test API. The message also
points to the README file that has more information.

BUG=509038

Change-Id: Ic5906a32aca5a4169184e07f6ca804170eb863c9
Reviewed-on: https://chromium-review.googlesource.com/858484Reviewed-by: Conley Owens <cco3@chromium.org>
Reviewed-by: Vincent Scheib <scheib@chromium.org>
Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531983}

third_party/WebKit/LayoutTests/external/wpt/bluetooth/README.md

@@ -9,6 +9,19 @@ the API to be loaded as needed.
 The Chromium implementation is provided by
 `../resources/chromium/web-bluetooth-test.js`.
 
+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
+using the `content_shell` executable.
+
+In the future, Chromium `src/device/bluetooth` may be refactored into a Mojo
+service. At this point, it would be possible to add the necessary testing hooks
+into stable Chrome without substantially increasing the binary size, similar to
+WebUSB.
+
+These bluetooth tests are upstreamed here because other browsers can reuse them
+by implementing the [Web Bluetooth Testing API], even if only on their internal
+infrastructure.
+
 [Web Bluetooth Testing API]: https://docs.google.com/document/d/1Nhv_oVDCodd1pEH_jj9k8gF4rPGb_84VYaZ9IG8M_WY/
 
 # Generated gen-* files from generator.py
@@ -45,4 +58,4 @@ Usage:
 $ python generate.py
 ```
 
-More details documented in `generate.py`.
\ No newline at end of file
+More details documented in `generate.py`.

third_party/WebKit/LayoutTests/external/wpt/resources/chromium/web-bluetooth-test.js

@@ -377,4 +377,12 @@ class FakeRemoteGATTDescriptor {
   }
 }
 
-navigator.bluetooth.test = new FakeBluetooth();
+// If this line fails, it means that current environment does not support the
+// Web Bluetooth Test API.
+try {
+  navigator.bluetooth.test = new FakeBluetooth();
+} catch {
+    throw 'Web Bluetooth Test API is not implemented on this ' +
+        'environment. See the bluetooth README at ' +
+        'https://github.com/w3c/web-platform-tests/blob/master/bluetooth/README.md#web-bluetooth-testing';
+}