Fix a srcset fractional density bug

Since https://codereview.chromium.org/293423002/ fractional density descriptors
(e.g. "1.5x.gif 1.5x") get rounded down to an int for intrinsic size calculations.

This CL fixes that, and adds tests to prevent such regressions in the future.

BUG=379651

Review URL: https://codereview.chromium.org/308793011

git-svn-id: svn://svn.chromium.org/blink/trunk@175293 bbb929c8-8fbe-4397-9dbb-9b2b20218538