Fix 'gn check' errors in //ui/* targets.
Tested with the following configs:
Linux:
$ gn gen out_gn/Debug
$ gn check out_gn/Debug
Android:
$ gn gen --args='os="android" cpu_arch="arm"' out_gn_android/Debug
$ gn check out_gn_android/Debug
Chromeos:
$ gn gen --args='os="chromeos"' out_gn_chromeos/Debug
$ gn check out_gn_chromeos/Debug
BUG=367595, 376000
TEST=see above
R=brettw@chromium.org
TBR=sky@chromium.org
Review URL: https://codereview.chromium.org/
872243004
Cr-Commit-Position: refs/heads/master@{#314102}