ui: accessibility: remove -Wunused-function for Atk
ui: accessibility: remove -Wunused-function for Atk

Newer version of GLib define *get_instance_private symbols in
the G_DEFINE_TYPE macros. These symbols might be unused in cases
where a GObject class doesn't access its instances private data,
triggering -Wunused-function warnings.

BUG=504375
TEST=compile on linux with GLib >= 2.37

Review URL: https://codereview.chromium.org/1204293002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#336576}

hgl888/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1