IndexedDB: Use UnionTraits for IDBKeyData

Coming out of a review for an onion soup CL
(https://crrev.com/c/1265900), mek@ suggested I use a UnionTraits
declaration to handle IDBKey.data and its union type IDBKeyData.

This CL implements that for the current Chromium variant of the
IndexedDB struct traits.

Bug: 717812
Change-Id: I8b599be0218f3542bbfa0fd59efeef1e319df6f6
Reviewed-on: https://chromium-review.googlesource.com/c/1284390
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602024}
3 files changed