Snap non toplevel dialog windows to pixel boundary.

BUG=731309
TEST=manual. Start chrome with --force-device-scale-factor=1.6 and make sure javascript dialog is crisp.

Review-Url: https://codereview.chromium.org/2934883003
Cr-Commit-Position: refs/heads/master@{#485032}

components/constrained_window/BUILD.gn

@@ -37,6 +37,7 @@ static_library("constrained_window") {
   if (use_aura) {
     deps += [
       "//ui/aura",
+      "//ui/compositor",
       "//ui/wm",
     ]
   }

components/constrained_window/DEPS

@@ -4,6 +4,7 @@ include_rules = [
   "+content/public/browser",
   "+ui/aura",
   "+ui/base",
+  "+ui/compositor",
   "+ui/display",
   "+ui/gfx",
   "+ui/views",

components/constrained_window/constrained_window_views.cc

@@ -24,6 +24,11 @@
 #import "components/constrained_window/native_web_contents_modal_dialog_manager_views_mac.h"
 #endif
 
+#if defined(USE_AURA)
+#include "ui/aura/window.h"
+#include "ui/compositor/dip_util.h"
+#endif
+
 using web_modal::ModalDialogHost;
 using web_modal::ModalDialogHostObserver;
 
@@ -126,6 +131,19 @@ void UpdateModalDialogPosition(views::Widget* widget,
   }
 
   widget->SetBounds(gfx::Rect(position, size));
+
+#if defined(USE_AURA)
+  if (!widget->is_top_level()) {
+    // Toplevel windows are automatiacally snapped, but CHILD windows
+    // may not. If it's not toplevel, snap the widget's layer to pixel
+    // based on the parent toplevel window, which should be snapped.
+    gfx::NativeView window = widget->GetNativeView();
+    views::Widget* toplevel =
+        views::Widget::GetTopLevelWidgetForNativeView(window->parent());
+    ui::SnapLayerToPhysicalPixelBoundary(toplevel->GetLayer(),
+                                         widget->GetLayer());
+  }
+#endif
 }
 
 }  // namespace