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This adds the HistoryBookmarkModel interface, which contains the functions
used by history. This interfaces is RefCountedThreadSafe as history holds a
reference to it from a background thread. The concrete implementation of
this is UrlIndex. UrlIndex may outlive BookmarkModel.

BookmarkModel::BlockUntilLoaded moves to a separate class, ModelLoader.
ModelLoader may also outlive BookmarkModel. BookmarkModel currently creates
and holds a reference the ModelLoader. I may change that, but this is a
good point to land code.

BUG=680698
TEST=covered by existing tests

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Id5d0bade960eaa76668213b9d221eba0a688b16d
Reviewed-on: https://chromium-review.googlesource.com/1039151
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563426}