Fix nullptr crash in IsSiteMuted

This CL adds a nullptr check in IsSiteMuted to prevent a crash on Mac.

Bug: 797647
Change-Id: Ic36f0fb39f2dbdf49d2bec9e548a4a6e339dc9a2
Reviewed-on: https://chromium-review.googlesource.com/848245Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526825}

chrome/browser/ui/tabs/tab_utils.cc

@@ -452,6 +452,13 @@ void SetSitesMuted(const TabStripModel& tab_strip,
 
 bool IsSiteMuted(const TabStripModel& tab_strip, const int index) {
   content::WebContents* web_contents = tab_strip.GetWebContentsAt(index);
+
+  // TODO(steimel): Why was this not a problem for AreAllTabsMuted? Is this
+  // going to be a problem for SetSitesMuted?
+  // Prevent crashes with null WebContents (https://crbug.com/797647).
+  if (!web_contents)
+    return false;
+
   GURL url = web_contents->GetLastCommittedURL();
 
   // chrome:// URLs don't have content settings but can be muted, so just check