Document SkMatrix default constructs to identity

SkMatrix has a constexpr default constructor which initializes to
identity. This allows it to be both efficient and always initialized.
However, the pre-C++11 documentation stating that SkMatrix starts off
uninitialized still exists. Update the documentation.

Change-Id: I4f81209ea82f0cef04ca9369ccfdf8e9f448ff71
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/432257
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed