Fix C++20 build.

std::string::max_size() is constexpr in C++20, which lets the compiler
determine that a comparison can never succeed.  Cast to prevent a
resulting warning.

Bug: 1284275
Change-Id: Ib56f3a9a114b9f3d2210df7846869f9452a60a58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3649750
Reviewed-by: Satoru Takabayashi <satorux@chromium.org>
Commit-Queue: Satoru Takabayashi <satorux@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1004522}