Adds Server Previews protocol UMA plus UMA for not accepting previews

components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc b/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
index 52cf1328dd087e952e599e1df148d8ee8fb3c09a..f1e7cf069f714149131ab807c44cc3fb8d413261 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
@@ -42,6 +42,7 @@ const char kEmptyImageDirective[] = "empty-image";
 const char kLitePageDirective[] = "lite-page";
 const char kCompressedVideoDirective[] = "compressed-video";
 const char kIdentityDirective[] = "identity";
+const char kPagePoliciesEmptyImageDirective[] = "page-policies=empty-image";
 
 // The legacy Chrome-Proxy response header directive for LoFi images.
 const char kLegacyChromeProxyLoFiResponseDirective[] = "q=low";
@@ -156,6 +157,10 @@ const char* identity_directive() {
   return kIdentityDirective;
 }
 
+const char* chrome_proxy_page_policies_empty_image_directive() {
+  return kPagePoliciesEmptyImageDirective;
+}
+
 const char* chrome_proxy_experiment_force_lite_page() {
   return kChromeProxyExperimentForceLitePage;
 }