Cache user agent string in FrameLoaderClientImpl instead of BlinkPlatformImpl

This CL makes FrameLoaderClientImpl cache user agent string instead of
BlinkPlatformImpl to avoid threading issue.

BUG=598479

Review URL: https://codereview.chromium.org/1839023002

Cr-Commit-Position: refs/heads/master@{#383915}

content/child/blink_platform_impl.cc

@@ -427,12 +427,7 @@ blink::WebSocketHandle* BlinkPlatformImpl::createWebSocketHandle() {
 }
 
 WebString BlinkPlatformImpl::userAgent() {
-  CR_DEFINE_STATIC_LOCAL(
-      blink::WebString, user_agent,
-      (blink::WebString::fromUTF8(GetContentClient()->GetUserAgent())));
-  DCHECK(user_agent ==
-         blink::WebString::fromUTF8(GetContentClient()->GetUserAgent()));
-  return user_agent;
+  return blink::WebString::fromUTF8(GetContentClient()->GetUserAgent());
 }
 
 WebData BlinkPlatformImpl::parseDataURL(const WebURL& url,

third_party/WebKit/Source/web/FrameLoaderClientImpl.cpp

@@ -748,7 +748,9 @@ String FrameLoaderClientImpl::userAgent()
     if (!override.isEmpty())
         return override;
 
-    return Platform::current()->userAgent();
+    if (m_userAgent.isEmpty())
+        m_userAgent = Platform::current()->userAgent();
+    return m_userAgent;
 }
 
 String FrameLoaderClientImpl::doNotTrackValue()

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());