Adds Server Previews protocol UMA plus UMA for not accepting previews

components/data_reduction_proxy/content/browser/content_lofi_decider_unittest.cc

Index: components/data_reduction_proxy/content/browser/content_lofi_decider_unittest.cc
diff --git a/components/data_reduction_proxy/content/browser/content_lofi_decider_unittest.cc b/components/data_reduction_proxy/content/browser/content_lofi_decider_unittest.cc
index 5caa2a4365c3764a27243332976446d258f7795a..aaec299877ae84690332ec066c5daf8bf606cf2f 100644
--- a/components/data_reduction_proxy/content/browser/content_lofi_decider_unittest.cc
+++ b/components/data_reduction_proxy/content/browser/content_lofi_decider_unittest.cc
@@ -771,11 +771,11 @@ TEST_F(ContentLoFiDeciderTest, VideoDirectiveDoesNotOverride) {
   std::unique_ptr<net::URLRequest> request =
       CreateRequestByType(content::RESOURCE_TYPE_MEDIA, false, true);
   net::HttpRequestHeaders headers;
-  headers.SetHeader(chrome_proxy_accept_transform_header(), "foo");
+  headers.SetHeader(chrome_proxy_accept_transform_header(), "empty-image");
   NotifyBeforeSendHeaders(&headers, request.get(), true);
   std::string header_value;
   headers.GetHeader(chrome_proxy_accept_transform_header(), &header_value);
-  EXPECT_EQ("foo", header_value);
+  EXPECT_EQ("empty-image", header_value);
 }
 
 TEST_F(ContentLoFiDeciderTest, IsSlowPagePreviewRequested) {