gfx: Add an implicit Insets to InsetsF conversion operator.

BUG=none
R=danakj@chromium.org

Review URL: https://codereview.chromium.org/102253009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243131 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243131