Chrome OS: Fix BootTimesRecorder crash on renderer crash.

This CL adds NULL checks to BootTimesRecorder so that it won't crash
on renderer crash.

TBR=oshima@chromium.org

Bug: 768446
Change-Id: I989b4096b452da0c92765143f05139413deb10b8
Reviewed-on: https://chromium-review.googlesource.com/728920Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510209}

chrome/browser/chromeos/boot_times_recorder.cc

@@ -66,7 +66,15 @@ RenderWidgetHost* GetRenderWidgetHost(NavigationController* tab) {
 }
 
 const std::string GetTabUrl(RenderWidgetHost* rwh) {
+  // rwh is null during initialization and shut down.
+  if (!rwh)
+    return std::string();
+
   RenderWidgetHostView* rwhv = rwh->GetView();
+  // rwhv is null if renderer has crashed.
+  if (!rwhv)
+    return std::string();
+
   for (auto* browser : *BrowserList::GetInstance()) {
     for (int i = 0, tab_count = browser->tab_strip_model()->count();
          i < tab_count;