WebScrollbarThemeClientImpl inherited DisplayItemClient from
ScrollbarThemeClient but it is never used as a DisplayItemClient.

Let Scrollbar instead of ScrollbarThemeClient inherit
DisplayItemClient, and use Scrollbar type in ScrollbarTheme
paint methods.

This avoids the need to let WebScrollbarThemeClientImpl follow
the rules of DisplayItemClient caching.

Review-Url: https://codereview.chromium.org/2045963003
Cr-Commit-Position: refs/heads/master@{#398818}