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This change adds a thrown error message for browser environments
that do not implement Web Bluetooth Test API. The message also
points to the README file that has more information.

BUG=509038

Change-Id: Ic5906a32aca5a4169184e07f6ca804170eb863c9
Reviewed-on: https://chromium-review.googlesource.com/858484Reviewed-by: Conley Owens <cco3@chromium.org>
Reviewed-by: Vincent Scheib <scheib@chromium.org>
Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531983}