Improves code readability in in core/html by

Using C++11 range-based loop.
Using ">>" instead of "> >" in templates.
Using nullptr instead of 0.


BUG=none
TEST=no layout test failures

Review URL: https://codereview.chromium.org/656723005

git-svn-id: svn://svn.chromium.org/blink/trunk@184834 bbb929c8-8fbe-4397-9dbb-9b2b20218538