More Blimp browser tests for navigation

blimp/engine/session/tab.cc

Index: blimp/engine/session/tab.cc
diff --git a/blimp/engine/session/tab.cc b/blimp/engine/session/tab.cc
index c1a01616296319466c365de07a68b6099e8799a3..69d5db8a16068794ff0308dafc8ea5f04f241119 100644
--- a/blimp/engine/session/tab.cc
+++ b/blimp/engine/session/tab.cc
@@ -75,11 +75,17 @@ void Tab::LoadUrl(const GURL& url) {
 }
 
 void Tab::GoBack() {
+  if (!web_contents_->GetController().CanGoBack()) {
+    return;
+  }
   DVLOG(1) << "Back in tab " << tab_id_;
   web_contents_->GetController().GoBack();
 }
 
 void Tab::GoForward() {
+  if (!web_contents_->GetController().CanGoForward()) {
+    return;
+  }
   DVLOG(1) << "Forward in tab " << tab_id_;
   web_contents_->GetController().GoForward();
 }