VR: fix the position of dialogs

Backplane of the hosted dialogs are moved to align
with main content. As a result the dialog itself
will be in the center of the web content.

BUG=808560

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_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: I6addedbfef905c51e11d5a6a5c95f889775a2078
Reviewed-on: https://chromium-review.googlesource.com/953247Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Amirhossein Simjour <asimjour@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541613}

chrome/browser/vr/ui_scene_creator.cc

@@ -634,7 +634,7 @@ void UiSceneCreator::CreateHostedUi() {
   backplane->SetDrawPhase(kPhaseForeground);
   backplane->SetName(kHostedUiBackplane);
   backplane->SetSize(kSceneSize, kSceneSize);
-  backplane->SetTranslate(0.0, 0.0, -kContentDistance);
+  backplane->SetTranslate(0.0, kContentVerticalOffset, -kContentDistance);
   EventHandlers event_handlers;
   event_handlers.button_up = base::BindRepeating(
       [](Model* model, UiBrowserInterface* browser) {