[Blimp] Creates engine tab class to handle tab related operations.

blimp/engine/session/page_load_tracker.cc

Index: blimp/engine/session/page_load_tracker.cc
diff --git a/blimp/engine/session/page_load_tracker.cc b/blimp/engine/session/page_load_tracker.cc
index 6f248ea119f9a4c99e8d66c79b60e710be67ea10..24e3a4871aeb134c601a3479d1120608bf7fed9a 100644
--- a/blimp/engine/session/page_load_tracker.cc
+++ b/blimp/engine/session/page_load_tracker.cc
@@ -24,6 +24,7 @@ content::RenderWidgetHost* GetRenderWidgetHostIfMainFrame(
 PageLoadTracker::PageLoadTracker(content::WebContents* web_contents,
                                  PageLoadTrackerClient* client)
     : client_(client) {
+  DCHECK(web_contents);
   Observe(web_contents);
 }