Add scheib to content/shell/OWNERS for bluetooth files.

This enables ownership of content shell support for testing
Web Bluetooth. An example of early support being added in
https://codereview.chromium.org/1132943002
content/shell/browser/layout_test/layout_test_bluetooth_adapter_provider.h/cc

BUG=420275

Review URL: https://codereview.chromium.org/1142523003

Cr-Commit-Position: refs/heads/master@{#329957}

content/shell/OWNERS

 mkwst@chromium.org
 pfeldman@chromium.org
+
+per-file *bluetooth*=scheib@chromium.org
+