[DevTools] Speculative fix for ResetViewport crash

It seems that we can have no layer tree view, but a
visual_viewport_container_layer_, because we
set them up and wind down separately. Adding null-check
to fight the crash.

Note there is no stable repro for this crash, so it
might be something else.

Bug: 902413
Change-Id: I33dc8419f33ab11d8a49d0be543ac0c8119f9d19
Reviewed-on: https://chromium-review.googlesource.com/c/1334887Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608371}

third_party/blink/renderer/core/exported/web_view_impl.cc

@@ -3312,7 +3312,8 @@ void WebViewImpl::UpdateDeviceEmulationTransform() {
   // scales and fuzzy raster from the compositor, force all content to
   // pick ideal raster scales.
   visual_viewport_container_layer_->SetTransform(device_emulation_transform_);
-  layer_tree_view_->ForceRecalculateRasterScales();
+  if (layer_tree_view_)
+    layer_tree_view_->ForceRecalculateRasterScales();
 }
 
 PageScheduler* WebViewImpl::Scheduler() const {