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The issue when using multiple shared images was that a `ScopedAccess`
was destroyed after it's `SharedImageRepresentation`.
The code was similar to the following:

  struct Pair {
      unique_ptr<Representation> representation;
      unique_ptr<Access> access;
  };

  base::flat_map<Key, Pair> map;
  map.erase(some_iterator);

In the Pair destructor C++ guarantees that `access` is destroyed before
`representation` but `erase` can move one element over another, causing
the move-assignment operator to be called. In this case the defaulted
move-assignment would first move `representation` then `access`. Causing
incorrect member destruction order for the move-to object.

Thanks to senorblanco@ and amaiorano@ for figuring out this wasn't a
compiler bug by jumping in the flat_map internals!

Bug: chromium:1038210
Change-Id: Ida90804a9840d1acba96a37c7cad17cf6bcc46ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014927Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734956}