Refactor test gfx_util.

Separated smaller change from https://codereview.chromium.org/109193002/

+ Added EXPECT_BOXF_EQ to use floating point almost equal.
~ Renamed color_util.{h,cc} to gfx_util.{h,cc}.

BUG=

Review URL: https://codereview.chromium.org/143673002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246229 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246229