A thread safe CSS calc parser for sizes

The sizes attribute parsing requires calc() parsing and evaluation.
This CL adds a thread safe calc parser (for unsigned length values).

BUG=357586

Review URL: https://codereview.chromium.org/252743004

git-svn-id: svn://svn.chromium.org/blink/trunk@172992 bbb929c8-8fbe-4397-9dbb-9b2b20218538