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/848245
Reviewed-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}
  explicit LastMuteMetadata(content::WebContents* contents) {}

  explicit LastMuteMetadata(content::WebContents* contents) {}
