Long timezone names overflowed the timezone cache which had a static
length of 100. This uses dynamically allocated std::strings as backing
stores instead.

Bug: chromium:842085
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I3da474c8b7c530b0933018c6239021979c320043
Reviewed-on: https://chromium-review.googlesource.com/1064111Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53243}