Fix Skia FontCache platform data construction to use right font size

The skia version of FontCache::createFontPlatformData incorrectly used
fontDescription.computedSize instead of computedPixelSize even if sub-
pixel font scaling wasn't enabled. This could cause an entry with the
wrong size to be added to the FontCache.

BUG=304986
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/26272006

git-svn-id: svn://svn.chromium.org/blink/trunk@159122 bbb929c8-8fbe-4397-9dbb-9b2b20218538