Bug 908300 - rename shared TextUtils to FontSizeHelper, add docs, fix tests r=vingtetun
Bug 908300 - use overflow and mutation observer to resize header font
add centering logic to auto-resize
Implement the centering logic
adding test application for headers
patch feedback for gmarty
Add centering unit tests
Merge commit 'fcd684270b4dd8bd046d5c41a831aeb6bf6999fa' of github.com:gmarty/gaia into bug-983812-shift

Conflicts:
	apps/sharedtest/test/unit/font_size_utils_test.js
	shared/js/font_size_utils.js
	shared/style/headers.css