[vr] handle sizing to children and anchoring

Previously we would not relayout children if the size changed. This is
problematic for children that wish to anchor to their parent.

Bug: 821220
Cq-Include-Trybots: luci.chromium.try:linux_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I3cbc91fd9a4d4d398d8ef3eba5f42b1f577f46f2
Reviewed-on: https://chromium-review.googlesource.com/962972Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543361}

chrome/browser/vr/elements/ui_element.cc

@@ -692,9 +692,14 @@ void UiElement::DoLayOutChildren() {
     return;
   }
 
+  bool requires_relayout = false;
   gfx::RectF bounds;
   for (auto& child : children_) {
     gfx::SizeF size = child->ContributedSize();
+    if (child->x_anchoring() != NONE || child->y_anchoring() != NONE) {
+      DCHECK(!child->contributes_to_parent_bounds());
+      requires_relayout = true;
+    }
     if (!child->IsVisible() || size.IsEmpty() ||
         !child->contributes_to_parent_bounds()) {
       continue;
@@ -717,7 +722,13 @@ void UiElement::DoLayOutChildren() {
   bounds.Inset(-x_padding_, -y_padding_);
   bounds.set_origin(bounds.CenterPoint());
   local_origin_ = bounds.origin();
+  if (bounds.size() == GetTargetSize())
+    return;
+
   SetSize(bounds.width(), bounds.height());
+
+  if (requires_relayout)
+    LayOutChildren();
 }
 
 void UiElement::LayOutChildren() {

chrome/browser/vr/elements/ui_element_unittest.cc

@@ -72,10 +72,21 @@ TEST(UiElement, BoundsContainChildren) {
   grand_parent->set_padding(0.1, 0.2);
   grand_parent->AddChild(std::move(parent));
 
+  auto anchored = std::make_unique<UiElement>();
+  anchored->set_y_anchoring(BOTTOM);
+  anchored->set_contributes_to_parent_bounds(false);
+
+  auto* anchored_ptr = anchored.get();
+  grand_parent->AddChild(std::move(anchored));
+
   grand_parent->DoLayOutChildren();
   EXPECT_RECT_NEAR(
       gfx::RectF(-0.5f, 0.5f, 9.4f, 7.8f),
       gfx::RectF(grand_parent->local_origin(), grand_parent->size()), kEpsilon);
+
+  gfx::Point3F p;
+  anchored_ptr->LocalTransform().TransformPoint(&p);
+  EXPECT_FLOAT_EQ(-3.9, p.y());
 }
 
 TEST(UiElement, BoundsContainScaledChildren) {