Move LineWidth.{h,cpp} into rendering/line

We introduced the 'line' subdirectory in rendering earlier. LineWidth should be in this directory.

Review URL: https://codereview.chromium.org/113033003

git-svn-id: svn://svn.chromium.org/blink/trunk@164191 bbb929c8-8fbe-4397-9dbb-9b2b20218538