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ModelLoader is a ref counted type, and its first reference used to be
taken in its constructor through base::BindOnce. The reference was
passed to a task runner, and released after the task has run.

However, if the PostTask failed or the posted task ran soon before the
construction had completed, the ModelLoader instance can be destroyed
before another reference is made on the original sequence. So,
`new ModelLoader` can return a stale pointer.

This CL adds a static constructor to ModelLoader, and makes the first
reference on the original sequence before passing a reference to the
other sequence.

Bug: 866456
Change-Id: I4d3c954ca39b7187fbd651c498e17273024c9968
Reviewed-on: https://chromium-review.googlesource.com/1151173
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578537}