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This fixes a regression in r298758 where if a 64-bit plugin was loaded after
a 32-bit plugin, and the versions were the same, then the 32-bit plugin would
be removed from the list, the 64-bit plugin plugin would be removed and
neither plugin would successfully load.

Solution is to only load the correct architecture plugins from the registry.

BUG=428549

Review URL: https://codereview.chromium.org/674073003

Cr-Commit-Position: refs/heads/master@{#302713}