DevTools: add emulation toggle button and splash screen.

BUG=327641
R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/347583003

git-svn-id: svn://svn.chromium.org/blink/trunk@176425 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unneeded checks in RenderTreeBuilder

Remove unneeded checks in RenderTreeBuilder, the same check is done in RenderTreeBuilder::shouldCreateRenderer.

Review URL: https://codereview.chromium.org/334263007

git-svn-id: svn://svn.chromium.org/blink/trunk@176426 bbb929c8-8fbe-4397-9dbb-9b2b20218538