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BookmarkModel can only be used from the UI thread, so this patch
enforces that with DCHECKs by adopting base/sequence_checker.h.

Such enforcement surfaces a few questionable usages in
chrome/browser/android/provider and corresponding tests, both of which
are updated. Most importantly, bookmarks::ModelLoader is plumbed
separately because it's thread safe, whereas access to BookmarkModel
is strictly limited to the UI thread.

Bug: None
Change-Id: I70e526e42982faf9e17d7d54f4a71eed34e2d5f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903130Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720861}