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This CL splits ScrollView::updateScrollbars() into 3 independent helper
functions, namely:

computeScrollbarExistence() computes whether scrollbars are needed given
the current ScrollView state.

adjustScrollbarExistence() creates/removes scrollbars and invoke appropriate
callbacks to update layout (if applicable).

updateScrollbarGeometry() positions the scrollbars and pushes the scroll
extents.

Also we no longer do update scrollbar passes in recursion. Instead we do
it in a loop and early exits if updateScrollbars() is re-entered.

No tests as there should be no behavior difference.

Review URL: https://codereview.chromium.org/314583008

git-svn-id: svn://svn.chromium.org/blink/trunk@175852 bbb929c8-8fbe-4397-9dbb-9b2b20218538