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Also update callers to use better way to construct the path using
StringPiece-s, avoiding string copy (GURL::path()) and substring
copy (std::string::substr(1)).

Move ExtensionIconSet's unittest to extensions/

BUG=None

Review-Url: https://codereview.chromium.org/2626923002
Cr-Commit-Position: refs/heads/master@{#443032}