Replace setIndexedDBClientCreateFunction madness with Supplements

IndexedDBClient was a special snowflake, created on demand using
a function pointer. Meanwhile, all of the other module clients
(that give access to web/ features like content settings) are
implemented using Supplements on Frame/Worker and minted during
context creation. This has led to bugs during unrelated refactoring
including races.

So just make IndexedDBClient a Supplement like the rest.

R=sigbjornf@opera.com,haraken@chromium.org
BUG=472754

Review URL: https://codereview.chromium.org/1865213006

Cr-Commit-Position: refs/heads/master@{#387117}
13 files changed