Fix Fuchsia implementation of base::RandBytes() to allow empty buffer.

base::RandBytes() may be called with output_length=0 and all
implementations supported this case, except Fuchsia. On Fuchsia the
function was crashing due to a failed CHECK(). This was breaking
MemoryDataSourceTest.EmptySource test.

Bug: 737802
Change-Id: Ib9ec20a82dd6966a146f0c66936e232c061466a5
Reviewed-on: https://chromium-review.googlesource.com/595012
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491086}

• base/rand_util_fuchsia.cc[diff]
• base/rand_util_unittest.cc[diff]