SolidColorScrollbarLayer's constructor takes a scroll_layer_id argument
similar to the other ScrollbarLayerInterface subclasses, but this
argument was ignored. This patch fixes this bug which lets us remove
some redundant code in tests.
BUG=693740
Review-Url: https://codereview.chromium.org/2734583002
Cr-Commit-Position: refs/heads/master@{#454894}