D3D: Update dynamic BufferUsage flags. Changed the mapping between gl::BufferUsage flags and D3DBufferUsage flags. Only gl::BufferUsage::DynamicDraw gets mapped to D3DBufferUsage::DYNAMIC now. This reflects a better mapping between GL and D3D. BUG=angleproject:3366 Change-Id: I5062f91fdb3664339e2c259b399d5f47401675d6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1569465 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>