String find has a faster overload for char, hence, we should pass a char
instead of a string where possible. Fixing clang-tidy warning.

Bug: v8:8015
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I49611d84bfb195992b6e2de538a726a8654b7b71
Reviewed-on: https://chromium-review.googlesource.com/1209348
Commit-Queue: Florian Sattler <sattlerf@google.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55713}