Do not duplicate allocation view entries when switching between views

.snapshot field is used to check if data source has changed or not before calling setDataSource on the DataGrid.

BUG=277984
R=alph@chromium.org

Review URL: https://codereview.chromium.org/189263008

git-svn-id: svn://svn.chromium.org/blink/trunk@168990 bbb929c8-8fbe-4397-9dbb-9b2b20218538