webui: fix clear browsing data visual glitch.
webui: fix clear browsing data visual glitch.

The bottom right/left corners aren't rounded because the gray bottom bar
is square and on top of the dialog's rounded corners.

Just inherit the border-radius selectively.

R=alekseys@chromium.org
BUG=none
TEST=visual

Review URL: https://codereview.chromium.org/913363002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#315924}

axinging/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,ltilve/chromium,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,hgl888/chromium-crosswalk,fujunwei/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,fujunwei/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,ltilve/chromium,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,ltilve/chromium,PeterWangIntel/chromium-crosswalk,Just-D/chromium-1,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,ltilve/chromium,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,ltilve/chromium,axinging/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,Chilledheart/chromium,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,ltilve/chromium,chuan9/chromium-crosswalk,fujunwei/chromium-crosswalk,fujunwei/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,ltilve/chromium,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,PeterWangIntel/chromium-crosswalk,Fireblend/chromium-crosswalk,fujunwei/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,fujunwei/chromium-crosswalk,Fireblend/chromium-crosswalk,Fireblend/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,fujunwei/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk,ltilve/chromium,Just-D/chromium-1,ltilve/chromium,Fireblend/chromium-crosswalk,Chilledheart/chromium,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk