The CMake file states c++17, Chromium is at c++17. This CL updates the
style guide documentation to specify c++17 as required for Tint.
Change-Id: I4dd8e9aa69de6f32a59ced2c5b861c83adf2edfe
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/85320
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>