webauthn: make caBLE's QRScanDialog class public.

I tightened up a lot of visibility in f50ddd34 but, while this class
isn't used directly from outside of the package, the Android system uses
reflection in ways that require it to be public.

BUG=1002262

Change-Id: Ie2c921e465e89de4f6ff650a240c0bebe799737c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548384
Auto-Submit: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#829263}

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

@@ -31,8 +31,10 @@ import java.nio.ByteBuffer;
 /**
  * Displays a preview of what the default (rear) camera can see and processes images for QR
  * codes. Closes once an applicable QR code has been found.
+ *
+ * (Needs to be public because of the way that the Android system works.)
  */
-class QRScanDialog extends DialogFragment implements Camera.PreviewCallback {
+public class QRScanDialog extends DialogFragment implements Camera.PreviewCallback {
     /**
      * FIDO QR codes begin with this prefix. This class will ignore QR codes that don't match
      * this.