Avoid use of CSSValue in MediaQueryExp and MediaQueryEvaluator

Since the new MediaQueryParser should be thread-safe and CSSPrimitiveValue may not be, MediaQueryExp should not generate a CSSValue when created by MediaQueryParser and SizesAttributeParser. 
This CL fixes that.

BUG=365605

Review URL: https://codereview.chromium.org/240453010

git-svn-id: svn://svn.chromium.org/blink/trunk@172457 bbb929c8-8fbe-4397-9dbb-9b2b20218538