Remove String forward-declaration in wtf/forward.h

This CL fixes the warning that declaration 'String' is never
referenced, but a declaration with the same name found in another
namespace 'v8' by `bugprone-forward-declaration-namespace`
clang-tidy check.

This removes String in wtf/forward.h to avoid this clang-tidy check,
instead manually forward declare it.

Bug: 1519029
Change-Id: I6e6026f3ae10d22bb1d8b0e4b3cebf32b9a96d32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5334322


Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1267353}