[dawn] Use absl::string_view where absl containers are used.

- Places that use absl::flat_hash_map/set cannot always
  implicitly convert std::string_view (i.e. in Google3), so
  for now, explicitly use absl::string_view.
- Conditionally includes an implicit converter from
  the Dawn internal StringView to absl::string_view, and then
  use absl::string_view instead of std::string_view when using
  absl utilities.

Change-Id: Iaa9fa7f7b1ff47073660520e3b84ce120b46d4c1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/208955
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>