Fix button focus on gtk
Fix button focus on gtk

We draw a custom focus indicator since the dotted focus indicator in
some gtk themes doesn't mix well with ImagePainter's scaling.

BUG=534280
R=erg@chromium.org

Review URL: https://codereview.chromium.org/1366773002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#350876}

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