[QrCode] Fixed camera in use issue after camera is released.

Bug: 1055288
Change-Id: Icbc384fbe92965eff27722c8ac095629a85a793b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2070974Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: Tanya Gupta <tgupta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744159}

chrome/browser/share/android/java/src/org/chromium/chrome/browser/share/qrcode/scan_tab/CameraPreview.java

@@ -82,6 +82,7 @@ public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback
 
         stopCameraPreview();
         mCamera.release();
+        mCamera = null;
 
         if (mCameraThread != null) {
             mCameraThread.quit();