[ScrollUnification] Correct LTHI test expectations

This CL makes it so that all tests in layer_tree_host_impl_unittest.cc
pass when the scroll unification flag is turned on. A followup
mechanical CL adds parameterized test suites to ensure it stays that
way.

The main changes here are that under unification, cases where
ScrollBegin would previously cause a scroll to be routed to the main
thread are now handled on the impl thread. In cases where the hit test
was unreliable or had a non-fast scrollable region, the return now
requests a main thread hit test. For simple main thread scrolling
reasons, a hit test isn't necessary.

Also made some minor updates to a few tests to ensure they're running
under the most realistic scenario by making sure they setup the viewport
correctly.

Bug: 1086625
Change-Id: I20fbd4c431c583e0a63241e91a3adaeb298bed0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2222262
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774012}

cc/test/property_tree_test_utils.cc

@@ -155,6 +155,7 @@ ScrollNode& CreateScrollNodeInternal(LayerType* layer,
   node->scrollable = !scroll_container_bounds.IsEmpty();
   node->user_scrollable_horizontal = true;
   node->user_scrollable_vertical = true;
+  node->is_composited = true;
 
   DCHECK(layer->has_transform_node());
   node->transform_id = layer->transform_tree_index();

cc/trees/layer_tree_host_impl.h

@@ -1027,7 +1027,7 @@ class CC_EXPORT LayerTreeHostImpl : public InputHandler,
   // |viewport_point|. If no layer is hit, this falls back to the inner
   // viewport scroll node. Returns:
   // - If |hit_test_sucessful| is false, hit testing has failed and the
-  //   compositor cannot determine the corerct scroll node (e.g. see comments in
+  //   compositor cannot determine the correct scroll node (e.g. see comments in
   //   IsInitialScrollHitTestReliable). |scroll_node| is always nullptr in this
   //   case.
   // - If |hit_test_successful| is true, returns the ScrollNode to use in