Revert accidentally committed newtab_button_mask.png
Revert accidentally committed newtab_button_mask.png

I only meant to update the copy in chrome/app/theme/ash, not the standard version.

BUG=none
TEST=none
TBR=ben@chromium.org

Review URL: https://chromiumcodereview.appspot.com/9968096

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@130468 0039d316-1c4b-4281-b951-d872f2087c98

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