Make ImageLoader non-ResourceClient

This CL makes ImageLoader non-ResourceClient, in order to remove classes that
inherit both of ResourceClient and ImageResourceObserver.

For such a class, whether ResourceClient::notifyFinished() is called before
ImageResourceObserver::imageChanged() depends on whether it hits MemoryCache
and whether addClient() is called before addObserver().

BUG=587663

Review URL: https://codereview.chromium.org/1844603002

Cr-Commit-Position: refs/heads/master@{#386015}
4 files changed