Ignore strict-transport-security header in synthetic response

In the experiment, we observed `strict-transport-security` headers are
sometimes inconsistent. It's fine to ignore this header in the synthetic
response, since the header is already handled by the networking layer.

Bug: 379764811
Change-Id: Ib523ab5b60805bf621cc8338d68a8574dfafe587
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6857078
Reviewed-by: Keita Suzuki <suzukikeita@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1502586}