Reland fast tab closure behind a flag

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 72825e9cde841fb966f64ce42073ceb65d3097fb..967983723339c39b65976e077286cf97d44a6b96 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -552,6 +552,11 @@ const char kExtensionsInActionBox[]         = "extensions-in-action-box";
 // manifest.
 const char kExtensionsOnChromeURLs[]        = "extensions-on-chrome-urls";
 
+// Enable the fast unload controller, which speeds up tab/window close by
+// running a tab's onunload js handler independently of the GUI -
+// crbug.com/142458 .
+const char kEnableFastUnload[]         = "enable-fast-unload";
+
 // By default, cookies are not allowed on file://. They are needed for testing,
 // for example page cycler and layout tests. See bug 1157243.
 const char kEnableFileCookies[]             = "enable-file-cookies";