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When BackupRefPtr is used for WebUIImpl::web_contents_, the destruction order of the fields causes a null dereference.
This is because BackupRefPtr is cleared in the destructor.
The destructor of WebUIMessageHandler subclasses are accessing the WebContentsImpl through WebUIImpl::web_contents_.

Bug: 1080832
Change-Id: I24570aad42dbd09febd94ef6d25be31c9817c091
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2564979Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834108}