Convert CHECK to NOTREACHED.

This CHECK was inserted for debugging purposes, but it's fine to
convert it to a NOTREACHED to avoid crashing Android SI users in
the wild.

Bug: 877092
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I35791a7af641c2425935aa095a16faf44c8cfa20
Reviewed-on: https://chromium-review.googlesource.com/1220100
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590449}

cc/trees/layer_tree_host_common.cc

@@ -576,7 +576,7 @@ void CalculateDrawPropertiesInternal(
                    << inputs->root_layer->layer_tree_impl()
                           ->settings()
                           .is_layer_tree_for_subframe;
-        CHECK(false);
+        NOTREACHED();
       }
 
       draw_property_utils::UpdatePageScaleFactor(