Fix the 'n' keystroke when viewing a diff in chrome 40.
Fix the 'n' keystroke when viewing a diff in chrome 40.

The problem is that the y-coordinate passed to window.scrollTo()
is undefined.  Previously that meant top-of-page, in chrome 40 it
seems to mean bottom-of-page.  An app should not be passing undefined
for that argument anyway.

R=kalman@chromium.org
BUG=424312

Review URL: https://codereview.chromium.org/668243003

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