IndexedDB: Upgrade WebIndexKeys type to WebIDBIndexKeys

In preparation for moving content/renderer/ code to Blink,
make WebIndexKeys mirror IndexedDBIndexKeys:
- rename to WebIDBIndexKeys to mirror WebIDB*
- move to web_idb_key.h to mirror indexed_db_key.h
- make this type a std::pair
- update code that used a variable of this type but not this type
  directly to now use this type, too

Bug: 717812
Change-Id: I132052654715201adbbec54a4b6877241db176a1
Reviewed-on: https://chromium-review.googlesource.com/c/1252912
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598647}
7 files changed