Add a field trial for the HTML preload scanner.

chrome/browser/chrome_content_browser_client.cc

Index: chrome/browser/chrome_content_browser_client.cc
diff --git a/chrome/browser/chrome_content_browser_client.cc b/chrome/browser/chrome_content_browser_client.cc
index 1ef1eba7a1a398bfbb51602cc81c3017356a4bfa..e68a5e9909fe979fcfb7bec3cd317a27fefaaf10 100644
--- a/chrome/browser/chrome_content_browser_client.cc
+++ b/chrome/browser/chrome_content_browser_client.cc
@@ -1186,6 +1186,9 @@ void MaybeAppendBlinkSettingsSwitchForFieldTrial(
     //       backgroundHtmlParserPendingTokenLimit
     "BackgroundHtmlParserTokenLimits",
 
+    // Keys: doHtmlPreloadScanning
+    "HtmlPreloadScanning",
+
     // Keys: lowPriorityIframes
     "LowPriorityIFrames",
   };