Fix skia_unittests under UBSan

Signed integer overflow is undefined, and the calls to rand() * 255 may
overflow. Since this was already nondeterministic with the srand(time())
call, just use base::RandBytes.

While I'm here, replace some &vec[0] with vec.data(). &vec[0] is UB when
vec is empty. vec is not empty here, but vec.data() captures the intent
of these functions more straightforwardly.

Bug: 1394755
Change-Id: If1ec10496ec9260930ec00f1325c905f9158a035
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4979785
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1216936}

• skia/ext/convolver_unittest.cc[diff]