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This makes NotificationTemplateBuilder own a WeakPtr rather than
a raw pointer to NotificationImageRetainer, which helps with tracking
potential memory issues.

Bug: 884224
Change-Id: I229ffd187ccd9b424987ab1473b240f898c06023
Reviewed-on: https://chromium-review.googlesource.com/1227717Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Commit-Queue: Xi Cheng <chengx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592759}