It turns out Aura (and Android) overlay scrollbars weren't using the minimum
defined in NativeThemeAura. Instead, the minimum was set from
ScrollbarTheme::minimumThumbLength which just returns the thumb thickness.

After adding an override for ScrollbarThemeOverlay which correctly uses the
value form NativeThemeAura, the only consumers of
ScrollbarTheme::minimumThumbLength are the mock classes so I moved the base
implementation into the Mock class and made the base pure virtual.

BUG=678595

Review-Url: https://codereview.chromium.org/2734483002
Cr-Commit-Position: refs/heads/master@{#460199}