Fix typo in //ui/base/BUILD.gn.
Fix typo in //ui/base/BUILD.gn.

This was broken in #339829.

TBR=brettw@chromium.org
NOTRY=true
BUG=504079

Review URL: https://codereview.chromium.org/1247353002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339878}

Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,axinging/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk