Add simple cache backend experiment hidden behind a flag.

chrome/browser/chrome_browser_field_trials.h

Index: chrome/browser/chrome_browser_field_trials.h
diff --git a/chrome/browser/chrome_browser_field_trials.h b/chrome/browser/chrome_browser_field_trials.h
index 72af0aeefbca3c4c59e2a17afaa99ccb5771abff..dc92241cdcee687d4496fe43661a74bbc334abf6 100644
--- a/chrome/browser/chrome_browser_field_trials.h
+++ b/chrome/browser/chrome_browser_field_trials.h
@@ -65,6 +65,9 @@ class ChromeBrowserFieldTrials {
   // TCP sockets on Windows instead of overlapped I/O.
   void WindowsOverlappedTCPReadsFieldTrial();
 
+  // A field trial to check the simple cache performance.
+  void SetUpSimpleCacheFieldTrial();
+
   // Instantiates dynamic trials by querying their state, to ensure they get
   // reported as used.
   void InstantiateDynamicTrials();