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The UseCounter being on the partial interface wasn't actually counting
anything. Move to the attribute instead.

Confirmed in V8CSSPartial generated code that this is now doing the
right thing.

Change-Id: Iab0f50535f7a06e4c2ff1172b7c80133554f9ae7
Reviewed-on: https://chromium-review.googlesource.com/923542Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537453}