Set default response for SimpleErrorBox and SimpleYesNoBox on Linux.
Set default response for SimpleErrorBox and SimpleYesNoBox on Linux.

- Make OK button default for SimpleErrorBox;
- Make Yes button default for SimpleYesNoBox;

This is inline with Windows behavior where the first button is the
default one, i.e. OK for SimpleErrorBox and Yes for SimpleYesNoBox.

BUG=chromium-os:15780
TEST=Verify fix for chromium-os:15780.

Review URL: http://codereview.chromium.org/7104003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@87506 0039d316-1c4b-4281-b951-d872f2087c98

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