Add missing |host_| check to DeferCommits in CompositorImplAndroid

We should have checked for |host_| before calling DeferCommits in
the last CL. Adding this to match previous behavior.

TBR=khushalsagar@chromium.org

Bug: 881706, 835353, 879656
Change-Id: I27a1e6fc67478b2e50c33ae48339adb58749b980
Reviewed-on: https://chromium-review.googlesource.com/1215084
Commit-Queue: Eric Karl <ericrk@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589782}

content/browser/renderer_host/compositor_impl_android.cc

@@ -1308,8 +1308,10 @@ bool CompositorImpl::HavePendingReadbacks() {
 std::unique_ptr<ui::CompositorLock> CompositorImpl::GetCompositorLock(
     ui::CompositorLockClient* client,
     base::TimeDelta timeout) {
+  std::unique_ptr<cc::ScopedDeferCommits> scoped_defer_commits =
+      host_ ? host_->DeferCommits() : nullptr;
   return lock_manager_.GetCompositorLock(client, timeout,
-                                         host_->DeferCommits());
+                                         std::move(scoped_defer_commits));
 }
 
 bool CompositorImpl::IsDrawingFirstVisibleFrame() const {