Fix an explicit-constructor presubmit warning

In the layer impl unittest.

Bug: None
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I5b1de75269b30c160e1ffc369aff09a353d8ca10
Reviewed-on: https://chromium-review.googlesource.com/685259
Commit-Queue: Peter Mayo <petermayo@chromium.org>
Reviewed-by: Peter Mayo <petermayo@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504470}

cc/layers/layer_impl_unittest.cc

@@ -486,7 +486,7 @@ class LayerImplScrollTest : public testing::Test {
  public:
   LayerImplScrollTest() : LayerImplScrollTest(LayerTreeSettings()) {}
 
-  LayerImplScrollTest(const LayerTreeSettings& settings)
+  explicit LayerImplScrollTest(const LayerTreeSettings& settings)
       : host_impl_(settings, &task_runner_provider_, &task_graph_runner_),
         root_id_(7) {
     host_impl_.active_tree()->SetRootLayerForTesting(