Add ClosedTabCache experiment flag

ClosedTabCache aims to make the "Reopen closed tab" button instant
for recently closed tabs.

Design doc:
https://docs.google.com/document/d/1SF230MYWgroe4WikDMn82ETd3-HFRzylKtIk0HZRaOU

BUG=1100946

Change-Id: I43934b0fe8b60ca9bbc09f27bbb62b38e33891d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274509Reviewed-by: Carlos Caballero <carlscab@google.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Sreeja Kamishetty <sreejakshetty@chromium.org>
Commit-Queue: Andreea Bacanu <aebacanu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784455}

chrome/browser/browser_features.cc

@@ -6,6 +6,11 @@
 
 namespace features {
 
+// Enables using the ClosedTabCache to instantly restore recently closed tabs
+// using the "Reopen Closed Tab" button.
+const base::Feature kClosedTabCache{"ClosedTabCache",
+                                    base::FEATURE_DISABLED_BY_DEFAULT};
+
 #if defined(OS_CHROMEOS)
 // Enables being able to zoom a web page by double tapping in Chrome OS tablet
 // mode.

chrome/browser/browser_features.h

@@ -16,6 +16,8 @@ namespace features {
 // All features in alphabetical order. The features should be documented
 // alongside the definition of their values in the .cc file.
 
+extern const base::Feature kClosedTabCache;
+
 #if defined(OS_CHROMEOS)
 extern const base::Feature kDoubleTapToZoomInTabletMode;
 #endif