Linux UI: Switch to the Gtk3 theme

Sets "use_gtk3 = true" by default.

BUG=79722
TBR=dpranke@chromium.org

> Review-Url: https://codereview.chromium.org/2670623002
> Cr-Commit-Position: refs/heads/master@{#452118}
> Committed: https://chromium.googlesource.com/chromium/src/+/872a494bba52e597388aec738d9d681183a3d47b

Review-Url: https://codereview.chromium.org/2670623002
Cr-Commit-Position: refs/heads/master@{#454491}
4 files changed