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The CL implements sticky section header rows in VPN page. Care is taken
to stack the rows with child controls that have layers below the sticky
section header rows.

BUG=664244
TEST=Run with --material-design-ink-drop-animation-speed=slow
     Touch a Wi-Fi connection row and while ripple is growing
     drag-scroll the list of connections up.
     Verify that the ripple is stacked below the Wi-Fi header row.

Review-Url: https://codereview.chromium.org/2557333003
Cr-Commit-Position: refs/heads/master@{#438689}