Makes views be initially hidden

This better matches what other toolkits do and lets the client
configure the view before we attempt to show something for it.

BUG=434429
TEST=covered by test
R=erg@chromium.org

Review URL: https://codereview.chromium.org/745743002

engine/src/flutter/tools/debugger/debugger.cc

@@ -59,6 +59,7 @@ void SkyDebugger::OnEmbed(
   content_ = mojo::View::Create(view_manager_);
   content_->SetBounds(root_->bounds());
   root_->AddChild(content_);
+  content_->SetVisible(true);
 
   window_manager_app_->InitFocus(
       scoped_ptr<mojo::FocusRules>(new mojo::BasicFocusRules(content_)));

engine/src/flutter/tools/tester/tester.cc

@@ -74,6 +74,7 @@ class SkyTester : public mojo::ApplicationDelegate,
     content_ = mojo::View::Create(view_manager_);
     content_->SetBounds(root_->bounds());
     root_->AddChild(content_);
+    content_->SetVisible(true);
 
     std::cout << "#READY\n";
     std::cout.flush();