Use C++11 features in core/page.

This CL improves code readability by:
- Replacing iterator loops with range based for loops.
- Using nullptr instead of 0.
- Using ">>" instead of "> >" in templates.
- Replacing typedef with "using".

BUG=none

Review URL: https://codereview.chromium.org/659493008

git-svn-id: svn://svn.chromium.org/blink/trunk@184654 bbb929c8-8fbe-4397-9dbb-9b2b20218538