IndexedDB: Stop using WebString and WebVector

A couple uses remain since other components in Blink use these
classes.

Bug: 717812
Change-Id: Ia32b6f2addb614d68a379809888edfbef46bca3a
Reviewed-on: https://chromium-review.googlesource.com/c/1361831
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616499}
39 files changed