Enable SiteEngagementEvictionPolicy behind --enable-site-eviction-policy flag.

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 b71448fd23161c26d445e108e69d5ed013d4bc87..2ec4ceace784536875a7696e72850f50586ac42a 100644
--- a/chrome/browser/chrome_content_browser_client.cc
+++ b/chrome/browser/chrome_content_browser_client.cc
@@ -39,6 +39,7 @@
 #include "chrome/browser/defaults.h"
 #include "chrome/browser/devtools/chrome_devtools_manager_delegate.h"
 #include "chrome/browser/download/download_prefs.h"
+#include "chrome/browser/engagement/site_engagement_eviction_policy.h"
 #include "chrome/browser/font_family_cache.h"
 #include "chrome/browser/geolocation/chrome_access_token_store.h"
 #include "chrome/browser/media/media_capture_devices_dispatcher.h"
@@ -142,6 +143,7 @@
 #include "net/ssl/ssl_cert_request_info.h"
 #include "ppapi/host/ppapi_host.h"
 #include "storage/browser/fileapi/external_mount_points.h"
+#include "storage/browser/quota/quota_manager.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/resources/grit/ui_resources.h"
@@ -1883,6 +1885,17 @@ ChromeContentBrowserClient::CreateQuotaPermissionContext() {
   return new ChromeQuotaPermissionContext();
 }
 
+void ChromeContentBrowserClient::AddQuotaEvictionPolicies(

[raymes, 2015/09/24 01:55:30]

I guess the alternate API would look like:
GetTemporaryStorageEvictionPolicy(context, quota_manager) 

This seems a bit clearer to me but I guess this partly related back to the
question of whether we ever expected there to be eviction policies for other
types.

If we did this, we could even return the LRU policy here when the site
engagement service is disabled and get rid of the map in QuotaManager (i.e.
always require the policy to be specified, no default policy). WDYT?

[calamity, 2015/10/26 03:01:57]

Done.

+    content::BrowserContext* context, storage::QuotaManager* quota_manager) {
+  if (SiteEngagementEvictionPolicy::IsEnabled()) {
+    quota_manager->SetQuotaEvictionPolicy(
+        storage::kStorageTypeTemporary,
+        make_scoped_ptr(
+            new SiteEngagementEvictionPolicy(storage::kStorageTypeTemporary,
+                                             quota_manager, context)));
+  }
+}
+
 void ChromeContentBrowserClient::AllowCertificateError(
     int render_process_id,
     int render_frame_id,