Fixed scrollbar margin for pinch virtual viewport.

Passed the scrollbar margin from the theme as the |trackStart|
parameter to ScrollingCoordinator::createSolidColorScrollbarLayer()

BUG=419911

Review URL: https://codereview.chromium.org/636733002

git-svn-id: svn://svn.chromium.org/blink/trunk@183478 bbb929c8-8fbe-4397-9dbb-9b2b20218538