Fix code around display overscan settings.

The current logic is wrong actually in case of updating overscan insets several times.
To replace the existing insets by new one, the existing insets has to be reimbursed beforehand.

BUG=139419


Review URL: https://chromiumcodereview.appspot.com/11140006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@162570 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed