Fixes #38060 and the following tests:

• tests/library/browsercontext-locale.spec.ts:
  • "should affect accept-language header"
• tests/library/browsercontext-set-extra-http-headers.spec.ts:
  • "should override extra headers from browser context"
• tests/library/defaultbrowsercontext-2.spec.ts:
  • "should support extraHTTPHeaders option"
• tests/library/popup.spec.ts:
  • "should inherit extra headers from browser context"
• tests/page/page-route.spec.ts:
  • "should show custom HTTP headers"
  • "should work with custom referer headers"
  • "should send referer"
• tests/page/page-set-extra-http-headers.spec.ts:
  • "should work"
  • "should work with redirects"
  • "should work with extra headers from browser context"
  • "should not duplicate referer header"