The function call overhead of the constructors and assignement operators
ended up being a hotspot even though the functions didn't do any
significant work.

BUG=angleproject:959

Change-Id: I96769879dabdbba7a222f98d87c5be0a829cb7dd
Reviewed-on: https://chromium-review.googlesource.com/262335Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>