Round off error while calculating the background-size: contain/cover.

While calculating the height and width of a background image for container
with the background-size: contain/cover, on some platforms 
these values(height and width) are not properly rounded off. Converting
these values to integer always takes the floor value rather than rounding it
off.

BUG=257402

Review URL: https://chromiumcodereview.appspot.com/23788011

git-svn-id: svn://svn.chromium.org/blink/trunk@158044 bbb929c8-8fbe-4397-9dbb-9b2b20218538