Fix null pointer crash.

There might not be a WebContents for a print job.

BUG=748368
TBR=avi@chromium.org

Change-Id: Ic6bc12aa69e7f97b1b7f30894fef2dc57144cbd6
Reviewed-on: https://chromium-review.googlesource.com/585247
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#489396}(cherry picked from commit 6fc1a19)
Reviewed-on: https://chromium-review.googlesource.com/587456
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/branch-heads/3163@{#65}
Cr-Branched-From: ff259ba-refs/heads/master@{#488528}

chrome/browser/printing/print_job_worker.cc

@@ -240,7 +240,7 @@ void PrintJobWorker::GetSettingsWithUI(
 
   // Running a dialog causes an exit to webpage-initiated fullscreen.
   // http://crbug.com/728276
-  if (web_contents->IsFullscreenForCurrentTab())
+  if (web_contents && web_contents->IsFullscreenForCurrentTab())
     web_contents->ExitFullscreen(true);
 
   // weak_factory_ creates pointers valid only on owner_ thread.