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Previously, a pointer to the NotificationImageRetainer object is stored in
NotificationTemplateBuilder. This is unnecessary as this object is only used
by two private methods called within Build() and never used again. This CL
changed to pass the pointer into those functions directly.

Separately, a GetCleanupTask() method is added for NotificationImageRetainer
so that it doesn't need to expose AsWeakPtr() to the public.

Bug: 888276
Change-Id: I4a7fa2a39e30c739f9adb7021b4809a4b56062fa
Reviewed-on: https://chromium-review.googlesource.com/c/1292712
Commit-Queue: Xi Cheng <chengx@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602496}