Add Click to Open PDF Placeholder feature

chrome/common/chrome_features.cc

Index: chrome/common/chrome_features.cc
diff --git a/chrome/common/chrome_features.cc b/chrome/common/chrome_features.cc
index 8613bab27189c0501b5cbdf592eec6cbf5f4eda1..c832ded2d998010c90fd66131c8a59d1d5543af6 100644
--- a/chrome/common/chrome_features.cc
+++ b/chrome/common/chrome_features.cc
@@ -110,6 +110,9 @@ const base::Feature kCaptureThumbnailOnLoadFinished{
 const base::Feature kCheckInstallabilityForBannerOnLoad{
     "CheckInstallabilityForBannerOnLoad", base::FEATURE_DISABLED_BY_DEFAULT};
 
+const base::Feature kClickToOpenPDFPlaceholder{
+    "ClickToOpenPDFPlaceholder", base::FEATURE_DISABLED_BY_DEFAULT};
+
 #if defined(OS_ANDROID)
 // Experiment to make Geolocation permissions in the omnibox and the default
 // search engine's search page consistent.