Don't use std::tuple in IndexRangeCache.
Don't use std::tuple in IndexRangeCache.

std::tuple currently isn't available on Chromium/OSX.

BUG=angle:773

Change-Id: I9f9efc88dcfe904e98a76df8b04086b88f724424
Reviewed-on: https://chromium-review.googlesource.com/238442
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

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