Initialize bitfield using constructor default member initializer for bit-field is a C++20 extension. This CL changes it to use constructor to avoid build failure. Bug: b/296433003 Change-Id: I33a45394644719b160f71eadca3a85a4d92f5c4e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4849554 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Charlie Lao <cclao@google.com>