Check for null iframe root layer in GraphicsLayerTreeBuilder

It's not clear why InCompositingMode() would be out of sync with
the allocation of the root graphics layer, even when the iframe is
throttled; but the CHECK was being hit.

BUG=1092673

Change-Id: I709f0958db7fb9e2c8fb315abac01c56aa0bded0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274674Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784086}

third_party/blink/renderer/core/paint/compositing/graphics_layer_tree_builder.cc

@@ -128,10 +128,10 @@ void GraphicsLayerTreeBuilder::RebuildRecursive(
       // state.
       DisableCompositingQueryAsserts disabler;
       if (inner_compositor->InCompositingMode()) {
-        GraphicsLayer* inner_root_layer = inner_compositor->RootGraphicsLayer();
-        // TODO(szager): Change to DCHECK after diagnosing crbug.com/1092673
-        CHECK(inner_root_layer);
-        layer_vector_for_children->push_back(inner_root_layer);
+        if (GraphicsLayer* inner_root_layer =
+                inner_compositor->RootGraphicsLayer()) {
+          layer_vector_for_children->push_back(inner_root_layer);
+        }
       }
       inner_compositor->ClearRootLayerAttachmentDirty();
     }