ui/gfx: Remove duplicated win condition.

We already have the same msvs_disabled_warnings declared at line 172. No
need to have it twice.

BUG=331829
TEST=gfx_unittests
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/555543003

Cr-Commit-Position: refs/heads/master@{#294756}