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This is needed as a prerequisite to making "visual viewport scrolls first"
the default. Scrolling the page before setting scale means that the layout
viewport is the only one that could scroll.

BUG=443724

Review URL: https://codereview.chromium.org/1301523006

git-svn-id: svn://svn.chromium.org/blink/trunk@201135 bbb929c8-8fbe-4397-9dbb-9b2b20218538