Always unregister placeholder image in canvas

It's possible that it has been disposed but its ID still exists. This
can lead to UAP bugs.

(cherry picked from commit 2df8ea19c5bba87209e452cea8473c05ea7d5250)

Bug: 978793
Change-Id: Idb4639cb78110878a28ab3c50807174343a66033
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1744642
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Aaron Krajeski <aaronhk@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Aaron Krajeski <aaronhk@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#685345}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752076
Cr-Commit-Position: refs/branch-heads/3809@{#1013}
Cr-Branched-From: d82dec1a818f378c464ba307ddd9c92133eac355-refs/heads/master@{#665002}
1 file changed