[Mac] Task manager alternating colors for background pages.
[Mac] Task manager alternating colors for background pages.

Previously all the background pages showed as a uniform color.  This
alternates like the regular table view, based on that color.

BUG=none
TEST=Alternating rows should continue the same pattern.

Review URL: http://codereview.chromium.org/6596090

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@76816 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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