Import StringPieceHash from chromium
Import StringPieceHash from chromium

Used in https://chromium-review.googlesource.com/c/365511/.

BUG=chromium:568664, chromium:604923

Change-Id: Id03a99809ccb3bebacf788a2fbf3a2c1e1b51c95
Reviewed-on: https://chromium-review.googlesource.com/365436
Reviewed-by: Robert Sesek <f55ad94e336f3a28322d0a25d280221ea9daf7a3@chromium.org>

chromium/mini_chromium,chromium/mini_chromium,chromium/mini_chromium