Remove dependency on ui::ScaleFactor from ui/gfx

As part of the work to removed dependencies on ui/base from ui/gfx I have
changed the public api to Canvas, ImageSkia, ImageSkiaRep and ImagePNGRep
to take float scale values instead of ui::ScaleFactor.

The notion of supported scale factors has been broken into 2 parts.
ui::SetSupportedScaleFactors remains and calls the
new ImageSkia::SetSupportedScales().

The initialization of the supported scale factors has been moved from layout.h
into ResourceBundle, and is done explicitly in tests that don't use
ResourceBundle.

BUG=103304
R=ben@chromium.org, oshima@chromium.org, sky@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=224473

Review URL: https://codereview.chromium.org/24175004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224876 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224876