Minor optimizations to DynamicHLSL. This makes us use std::ostringstream in more places, instead of string concatenation. BUG=chromium:697758 Change-Id: Ifdcaa2e7e119664fc9cfdc566ea13b519a294714 Reviewed-on: https://chromium-review.googlesource.com/521729 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>