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We can do this without any string copy, so use StringPiece
variant from GURL.

This is interesting because every extension resource fetch goes
through this method.

Bug: None
Change-Id: Ief332c09295b1087f64721e44f026eccc4a49497
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1631938Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664028}