Remove user agent from HeadlessBrowserContextOptions.

headless/lib/browser/headless_browser_context_impl.cc

Index: headless/lib/browser/headless_browser_context_impl.cc
diff --git a/headless/lib/browser/headless_browser_context_impl.cc b/headless/lib/browser/headless_browser_context_impl.cc
index 29c9da3f7472a1d45aad41708837a88bf816916e..2f4ebd08e2cdb03735fcdc102f016266c4ffc160 100644
--- a/headless/lib/browser/headless_browser_context_impl.cc
+++ b/headless/lib/browser/headless_browser_context_impl.cc
@@ -313,12 +313,6 @@ HeadlessBrowserContext::Builder::SetProtocolHandlers(
   return *this;
 }
 
-HeadlessBrowserContext::Builder& HeadlessBrowserContext::Builder::SetUserAgent(
-    const std::string& user_agent) {
-  options_->user_agent_ = user_agent;
-  return *this;
-}
-
 HeadlessBrowserContext::Builder&
 HeadlessBrowserContext::Builder::SetProductNameAndVersion(
     const std::string& product_name_and_version) {