Fix handling of float source-size in HTMLImageElement

At https://codereview.chromium.org/633643003/ I fixed the handling of float
source-size in HTMLSrcsetParser and SizesAttributeParser, but left out fixing
it in HTMLImageElement.
This CL fixes that.

BUG=418905

Review URL: https://codereview.chromium.org/641663002

git-svn-id: svn://svn.chromium.org/blink/trunk@183414 bbb929c8-8fbe-4397-9dbb-9b2b20218538