cablev2: add USBHandler on the phone side.

This code matches up with the Android accessory device code recently
added on the desktop side. It is not yet wired into anything on the
phone side.

BUG=1002262

Change-Id: I802eb6a0275a71744b374589003f5a0675bbb58d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341033
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#799884}

chrome/android/features/cablev2_authenticator/BUILD.gn

@@ -11,6 +11,7 @@ android_library("java") {
     "java/src/org/chromium/chrome/browser/webauth/authenticator/CableAuthenticatorUI.java",
     "java/src/org/chromium/chrome/browser/webauth/authenticator/CameraView.java",
     "java/src/org/chromium/chrome/browser/webauth/authenticator/QRScanDialog.java",
+    "java/src/org/chromium/chrome/browser/webauth/authenticator/USBHandler.java",
   ]
 
   deps = [

chrome/android/features/cablev2_authenticator/java/src/org/chromium/chrome/browser/webauth/authenticator/CableAuthenticator.java

@@ -153,6 +153,15 @@ class CableAuthenticator {
         mCallback.onComplete();
     }
 
+    /**
+     * Called by USBHandler to signal that a USB peer wrote data.
+     */
+    public byte[] onUSBWrite(byte[] payload) {
+        assert mTaskRunner.belongsToCurrentThread();
+        // TODO: wire up.
+        return null;
+    }
+
     // Calls from native code.
 
     /**