Correct addition of scheib, move from content/shell/OWNERS to ./browser/layout_test/OWNERS

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

This change corrects https://codereview.chromium.org/1142523003
which incorrectly used the per-file directive. per-file only applies
to files in the same directory as the OWNERS file.

BUG=420275

Review URL: https://codereview.chromium.org/1128423004

Cr-Commit-Position: refs/heads/master@{#329975}

content/shell/OWNERS

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

content/shell/browser/layout_test/OWNERS

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