Changed minimum stale previews staleness to 5 minutes

This is because non-stale previews can be served up to
five minutes after creation. The previous minimum was 2
minutes.

BUG=718643

Change-Id: I207707cbac5885ca72fc67980a495cc7eca3d2d1
Reviewed-on: https://chromium-review.googlesource.com/570828Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486581}

chrome/browser/previews/previews_infobar_delegate.cc

@@ -37,7 +37,7 @@ namespace {
 
 const char kMinStalenessParamName[] = "min_staleness_in_minutes";
 const char kMaxStalenessParamName[] = "max_staleness_in_minutes";
-const int kMinStalenessParamDefaultValue = 2;
+const int kMinStalenessParamDefaultValue = 5;
 const int kMaxStalenessParamDefaultValue = 1440;
 
 void RecordPreviewsInfoBarAction(