Only use subpixel-positioned text in UI when requested.

Update SkiaTextRenderer to explicitly take a
|subpixel_positioning| parameter. Previously, it requested
subpixel positioning whenever antialiasing was enabled,
which prevented users' strong-hinting settings from being
honored.

BUG=318395

Review URL: https://codereview.chromium.org/166673005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251580 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251580