Fix segfault in FlingSchedulerAndroid::ScheduleFlingProgress

GetRootWindow can return null but ScheduleFlingProgress doesn't check
for it.

Bug: 855045
Change-Id: Ib82b8169d74efc02800bd7b975f111f653bbadc3
Reviewed-on: https://chromium-review.googlesource.com/1110260
Commit-Queue: Saman Sami <samans@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569313}

content/browser/renderer_host/input/fling_scheduler_android.cc

@@ -29,7 +29,7 @@ void FlingSchedulerAndroid::ScheduleFlingProgress(
     // If the root window does not have a Compositor (happens on Android
     // WebView), we'll never receive an OnAnimate call. In this case fall back
     // to BeginFrames coming from the host.
-    if (!window->GetCompositor()) {
+    if (!window || !window->GetCompositor()) {
       host_->SetNeedsBeginFrameForFlingProgress();
       return;
     }