Make LauncherSearchIconImageLoader ref counted.

Get rid of deprecated linked_ptr usage. Use RefCounted because the
object has shared ownership.

BUG=556939

Change-Id: Iab75d2a4cf834227764d4ac1888e3efa690b2ce2
Reviewed-on: https://chromium-review.googlesource.com/1053992
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Yuki Awano <yawano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559422}
6 files changed