Enable gtk3 for 32-bit builds

Now that [1] has been fixed and libgtk-3-0 has been installed in
clusterfuzz's 32-bit configuration, it's safe to enable gtk3 for all
platforms.

[1] https://crbug.com/699669

BUG=79722
TBR=dpranke@chromium.org
CC=erg@chromium.org,inferno@chromium.org

Review-Url: https://codereview.chromium.org/2847863002
Cr-Commit-Position: refs/heads/master@{#468110}
1 file changed