IndexedDB: Mojom updates: rename files, update return types, move target

- Rename Mojom traits to indexeddb_mojom_traits.{cc,h}, from
  https://crrev.com/c/1265900/9/third_party/blink/renderer/modules/indexeddb/indexed_db_struct_traits.h.

- Update some non-built-in type return values to const-ref, from
  https://crrev.com/c/1265900/26/third_party/blink/renderer/modules/indexeddb/indexed_db_mojom_traits.h@30.

- Add a mojom_modules build target to allow a future Blink variant
  of the IndexedDB Mojo types to access symbols in the modules layer,
  from https://crrev.com/c/1265900.

Bug: 717812
Change-Id: I40f123cdee5bc7ac3102316d6cf20a191b8252a4
Reviewed-on: https://chromium-review.googlesource.com/c/1318374
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605562}
8 files changed