Make RenderStyle return const Length&'s instead of copying Lengths.

The Length copy constructor does a memcpy and a branch and is
showing up on profiles. There's no need to be copying around
Lengths in the vast majority of cases.

Review URL: https://codereview.chromium.org/189243002

git-svn-id: svn://svn.chromium.org/blink/trunk@168696 bbb929c8-8fbe-4397-9dbb-9b2b20218538