Added support for heavy page previews transformations

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 7c17395157bd08fef8f544ac3957e526f82ecf6f..375416a081c2eb889addd5dc3286b073f0ff56d7 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
@@ -43,6 +43,8 @@ const char kIdentityDirective[] = "identity";
 const char kChromeProxyLitePageIngoreBlacklistDirective[] =
     "exp=ignore_preview_blacklist";
 
+const char kIfHeavyQualifier[] = "if-heavy";
+
 const char kChromeProxyActionBlockOnce[] = "block-once";
 const char kChromeProxyActionBlock[] = "block";
 const char kChromeProxyActionBypass[] = "bypass";
@@ -139,6 +141,10 @@ const char* chrome_proxy_lite_page_ignore_blacklist_directive() {
   return kChromeProxyLitePageIngoreBlacklistDirective;
 }
 
+const char* if_heavy_qualifier() {
+  return kIfHeavyQualifier;
+}
+
 bool IsEmptyImagePreview(const net::HttpResponseHeaders& headers) {
   return IsPreviewType(headers, kEmptyImageDirective);
 }