IndexedDB: Remove observer transactions from frontend

These transactions are no longer supported in the backend and we've
planned to remove them from the frontend for some time.  This CL
updates OnChanges() to no longer create this type of transaction
and removes the corresponding codepaths from IDBTransaction.

Bug: 717812
Change-Id: I5bd807d11411ad18cab2737025647fc00230dae8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1526802
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642199}
3 files changed