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Proposed fix is to discard all unsupported representations (Read fractional scales) in the Extension icon image when
the icon is loaded. This ensures that stale caches are invalidated and the correct icons are loaded when the cache is regenerated.

BUG=461958

Review URL: https://codereview.chromium.org/1025513004

Cr-Commit-Position: refs/heads/master@{#322225}