Add scoped timer histogram helper macros. These macros replace a lot of repeated timing code, and mirror how Chromium operates. BUG=516027 Change-Id: Ib03e35d0f3b8638b9b0e8785c397c8381f16b041 Reviewed-on: https://chromium-review.googlesource.com/289995 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>