Remove legacy workaround for lack of std::unordered_map.

Chromium test (likely future roll PR):
https://chromium-review.googlesource.com/c/chromium/src/+/1137358

V8 test (likely future roll PR):
https://chromium-review.googlesource.com/c/v8/v8/+/1136796

Change-Id: I23b735d84548eada43a4414512a5fa062bb757de
13 files changed