[fuchsia] Update zx_cprng_draw
[fuchsia] Update zx_cprng_draw

This syscall can no longer fail to provide the requested number of
bytes, simplifying callers.

Change-Id: Id7f0af98a1ada087ad4f130ee9a1d34519eee521
Reviewed-on: https://dart-review.googlesource.com/62640
Reviewed-by: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>
Commit-Queue: Adam Barth <261ec7bc01029d75f3df176083f56735e31b8587@google.com>

dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk