PolicyManager: Include the Polling interval for kVariableModePoll variables.

A kVariableModePoll variable requires to be polled to know the if the
value changed. This patch adds a new GetPollInterval method that
tells how often a kVariableModePoll variable should be polled. This
value will be used by the policy manager to trigger re-evaluations
of the same policy request.

BUG=chromium:341209
TEST=unit tests added and passes.

Change-Id: I9c982ec8106ce39a0bc0889df7686add131b3bea
Reviewed-on: https://chromium-review.googlesource.com/187703
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>

policy_manager/variable.h

diff --git a/policy_manager/variable.h b/policy_manager/variable.h
index d4fba0c..373af7f 100644
--- a/policy_manager/variable.h
+++ b/policy_manager/variable.h
@@ -49,16 +49,42 @@
     return mode_;
   }
 
+  // For VariableModePoll variables, it returns the polling interval of this
+  // variable. In other case, it returns 0.
+  base::TimeDelta GetPollInterval() const {
+    return poll_interval_;
+  }
+
  protected:
+  // Creates a BaseVariable using the default polling interval (5 minutes).
   BaseVariable(const std::string& name, VariableMode mode)
-      : name_(name), mode_(mode) {}
+      : BaseVariable(name, mode,
+                     base::TimeDelta::FromMinutes(kDefaultPollMinutes)) {}
+
+  // Creates a BaseVariable with mode kVariableModePoll and the provided
+  // polling interval.
+  BaseVariable(const std::string& name, base::TimeDelta poll_interval)
+      : BaseVariable(name, kVariableModePoll, poll_interval) {}
 
  private:
+  BaseVariable(const std::string& name, VariableMode mode,
+               base::TimeDelta poll_interval)
+    : name_(name), mode_(mode),
+      poll_interval_(mode == kVariableModePoll ?
+                     poll_interval : base::TimeDelta()) {}
+
+  // The default PollInterval in minutes.
+  static constexpr int kDefaultPollMinutes = 5;
+
   // The variable's name as a string.
   const std::string name_;
 
   // The variable's mode.
   const VariableMode mode_;
+
+  // The variable's polling interval for VariableModePoll variable and 0 for
+  // other modes.
+  const base::TimeDelta poll_interval_;
 };
 
 // Interface to a Policy Manager variable of a given type. Implementation
@@ -82,6 +108,9 @@
   Variable(const std::string& name, VariableMode mode)
       : BaseVariable(name, mode) {}
 
+  Variable(const std::string& name, const base::TimeDelta& poll_interval)
+      : BaseVariable(name, poll_interval) {}
+
   // Gets the current value of the variable. The current value is copied to a
   // new object and returned. The caller of this method owns the object and
   // should delete it.