Fix subresource requests in ReduceAcceptLanguage OT when no restarts

This CL fixes subsequent subresource request sends the reduced
Accept-Language header when site initially opts-in the ReduceAcceptLanguage OT and no restart happens.

Update the commit reduce_accept_language when navigation request
commits if it's empty. This helps the subresource uses the commit
language within the first request when site initially opt-in the origin trial, and it doesn't require sites to resend the request.

Bug: 1383728
Change-Id: I2e0471882f64570cd9a92c74d76dcaf6cefa68b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4021994


Reviewed-by: Andrew Williams <awillia@chromium.org>
Commit-Queue: Victor Tan <victortan@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1072304}