Switch CustomHistogram::ArrayToCustomRanges to base::span<>

While at it, also rename the method to ArrayToCustomEnumRanges and
close out an old TODO.

TBR=hubbe@chromium.org, kbr@chromium.org
BUG=837308

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I56464a852707550dd625a1da30186aaaa9fdef38
Reviewed-on: https://chromium-review.googlesource.com/1055947
Commit-Queue: Ryan Sleevi <rsleevi@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558089}
11 files changed