Revert of Change minimum length of Aura overlay scrollbars. (patchset #9 id:160001 of https://codereview.chromium.org/2734483002/ )

Reason for revert:
Unit test failing on Android: https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Android%20%28Nexus4%29/builds/62927

Original issue's description:
> Change minimum length of Aura overlay scrollbars.
>
> 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}
> Committed: https://chromium.googlesource.com/chromium/src/+/151a2db45f5fdbca8973988dd51d2bf6c86fbc8c

TBR=estade@chromium.org,nasko@chromium.org,pdr@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=678595

Review-Url: https://codereview.chromium.org/2780923003
Cr-Commit-Position: refs/heads/master@{#460224}
12 files changed