Use StringConstant instead of NewStringFromAsciiChecked so strings are
deduplicated.

Change-Id: I0c5395be6d06caacd7d257b61bd2372da2fce427
Reviewed-on: https://chromium-review.googlesource.com/1025815Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52763}