Remove --background-color-instead-of-checkerboard flag.

This is now unused on all platforms.

R=aelias
BUG=350142

Review URL: https://codereview.chromium.org/189433003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255752 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255752