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To reduce GetDocument() usage.
As https://github.com/w3c/webappsec-csp/issues/146 has already been
closed and the Blink implemetation has been changed,
this UseCounter is no longer needed.

Bug: 878274
Change-Id: Icb55058d369b29992436229a9b971bc6f0b8f2a6
Reviewed-on: https://chromium-review.googlesource.com/c/1192303Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614762}