Add DCHECK that Reload should never be called with ReloadType::NONE

In ReloadType, NONE means that the navigation is not a reload, so it
makes no sense to call Reload function with ReloadType::NONE as
parameter.

This CL clarifies it by mentioning it in public API comment and add a
DCHECK in the implementation.

BUG=
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2740843004
Cr-Commit-Position: refs/heads/master@{#456129}

content/browser/frame_host/navigation_controller_impl.cc

@@ -297,6 +297,8 @@ void NavigationControllerImpl::Restore(
 
 void NavigationControllerImpl::Reload(ReloadType reload_type,
                                       bool check_for_repost) {
+  DCHECK_NE(ReloadType::NONE, reload_type);
+
   if (transient_entry_index_ != -1) {
     // If an interstitial is showing, treat a reload as a navigation to the
     // transient entry's URL.

content/public/browser/navigation_controller.h

@@ -356,6 +356,7 @@ class NavigationController {
   // cases pass in true in production code, but would do false for testing, or
   // in cases where no user interface is available for prompting.  If a
   // transient entry is showing, initiates a new navigation to its URL.
+  // NOTE: |reload_type| should never be NONE.
   virtual void Reload(ReloadType reload_type, bool check_for_repost) = 0;
 
   // Removing of entries -------------------------------------------------------