Extension docs build script, gyp target and PRESUBMIT.PY check

chrome/common/extensions/docs/js/bootstrap.js

+window.onload = function() {
+  // Regenerate page if we are passed the "?regenerate" search param
+  // or if the user-agent is chrome AND the document is being served
+  // from the file:/// scheme.
+  if (window.location.search == "?regenerate" ||
+      navigator.userAgent.indexOf("Chrome") > -1) {
+    // Hide body content initially to minimize flashing.
+    document.getElementsByTagName("body")[0].className = "hidden";
+    window.renderPage();
+  }
+}