Cache user agent string in FrameLoaderClientImpl instead of BlinkPlatformImpl

third_party/WebKit/Source/web/FrameLoaderClientImpl.h

Index: third_party/WebKit/Source/web/FrameLoaderClientImpl.h
diff --git a/third_party/WebKit/Source/web/FrameLoaderClientImpl.h b/third_party/WebKit/Source/web/FrameLoaderClientImpl.h
index 9ff9d19ce51f13cf6e322489a89db8d3e8ed1e01..80cf1c2c236a8560dd0dc699d81cd1cefb3ab415 100644
--- a/third_party/WebKit/Source/web/FrameLoaderClientImpl.h
+++ b/third_party/WebKit/Source/web/FrameLoaderClientImpl.h
@@ -192,6 +192,8 @@ private:
     // The WebFrame that owns this object and manages its lifetime. Therefore,
     // the web frame object is guaranteed to exist.
     RawPtrWillBeMember<WebLocalFrameImpl> m_webFrame;
+
+    String m_userAgent;
 };
 
 DEFINE_TYPE_CASTS(FrameLoaderClientImpl, FrameLoaderClient, client, client->isFrameLoaderClientImpl(), client.isFrameLoaderClientImpl());