Apply user agent exactly once for requests from Blink.

For most regular fetches, we apply the user agent twice:
- addAdditionalRequestHeaders
- prepareRequest

This patch changes FrameLoader::applyUserAgent to
FrameLoader::applyUserAgentIfNeeded which first checks if the user agent
is set. This avoids going to the embedder to calculate the override twice.

BUG=348655

Review-Url: https://codereview.chromium.org/2445653003
Cr-Commit-Position: refs/heads/master@{#427180}

third_party/WebKit/Source/core/loader/FrameFetchContext.cpp

@@ -248,8 +248,6 @@ void FrameFetchContext::addAdditionalRequestHeaders(ResourceRequest& request,
 
   if (frame()->settings() && frame()->settings()->dataSaverEnabled())
     request.setHTTPHeaderField("Save-Data", "on");
-
-  frame()->loader().applyUserAgent(request);
 }
 
 CachePolicy FrameFetchContext::getCachePolicy() const {