Fix a MediaValues::computeLength bug and add unit tests

MediaValues::computeLength had bug where the multiplication factor was an int, instead of a double.
That resulted in skewed computed values for viewport units.
This CL fixes that, and adds unit tests to make sure computeLength calculate length values correctly.

BUG=365787

Review URL: https://codereview.chromium.org/258613002

git-svn-id: svn://svn.chromium.org/blink/trunk@172514 bbb929c8-8fbe-4397-9dbb-9b2b20218538