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vertical and horizontal tracks together.
Update ScrollView to allow the native theme to paint the corner square when
both horizontal and vertical scrollbars are showing.
This is done by installing a view at the corner and have it call
NativeTheme::Paint(..).

BUG=401346

Review URL: https://codereview.chromium.org/462373002

Cr-Commit-Position: refs/heads/master@{#289483}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289483 0039d316-1c4b-4281-b951-d872f2087c98