Add explicit conversion for wgpu::StringView -> absl::string_view

- Adding this templated solution should be a more fail-safe fix
  w.r.t google3 code where std::string_view is not always
  available, and instead absl::string_view is used. The
  templated solution also allows us to avoid including absl
  in the header.

Change-Id: I8ada93abfb688b4762ffd5cf1a3dc5676f16b6c6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/211234
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: James Price <jrprice@google.com>
Commit-Queue: Loko Kung <lokokung@google.com>
Auto-Submit: Loko Kung <lokokung@google.com>