[session restore] CHECK that restored browsers have an active tab.

https://crbug.com/1032348 tells us that there are browsers with no
active tab in some cases when --restore-last-session is specified. To
get a better undertanding of the problem, this CL adds a CHECK to
get a crash report if a Browser doesn't have an active tab at the
end of SessionRestoreImpl::ProcessSessionWindows().

Bug: 1032348
Change-Id: Ic6919429ba9a1ece3a21230c0d63c9d2bb2a79bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1966523
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#725989}

chrome/browser/sessions/session_restore.cc

@@ -472,6 +472,11 @@ class SessionRestoreImpl : public BrowserListObserver {
       //    restore, if needed.
       if (close_active_tab)
         chrome::CloseWebContents(browser, active_tab, true);
+
+      // Sanity check: A restored browser should have an active tab.
+      // TODO(https://crbug.com/1032348): Change to DCHECK once we understand
+      // why some browsers don't have an active tab on startup.
+      CHECK(browser->tab_strip_model()->GetActiveWebContents());
     }
 
     if (browser_to_activate && browser_to_activate->is_type_normal())
@@ -531,7 +536,9 @@ class SessionRestoreImpl : public BrowserListObserver {
                             int initial_tab_count,
                             std::vector<RestoredTab>* created_contents) {
     DVLOG(1) << "RestoreTabsToBrowser " << window.tabs.size();
-    DCHECK(!window.tabs.empty());
+    // TODO(https://crbug.com/1032348): Change to DCHECK once we understand
+    // why some browsers don't have an active tab on startup.
+    CHECK(!window.tabs.empty());
     base::TimeTicks now = base::TimeTicks::Now();
     base::TimeTicks latest_last_active_time = base::TimeTicks::UnixEpoch();
     // The last active time of a WebContents is initially set to the