Add a descriptive message to NotFoundError in getUserMedia()

It is currently reporting a default message which is a bit misleading.

Bug: 721412
Change-Id: I8954eb8b433d70363b099543170d71ec0afb5037
Reviewed-on: https://chromium-review.googlesource.com/817559
Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522805}
1 file changed