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Allow Desktop Canvas to be scrolled out from under System UI.

This change adds the ability to scroll the desktop canvas out from under
any visible System UI.

When System UI is displayed (for Kitkat and higher) the
TouchInputHandler instance is called with the state/sizes of the System
UI per edge of the screen.  These values are passed to the DesktopCanvas
instance which uses them for determining whether the System UI overlaps
with the remote image content.  If there is overlap then it adjusts
the region of valid cursor and viewport positions which then allows
the user to pan the content out from 'under' the System UI which is
obscuring it.allowable position of the viewport.

When the System UI disappears (or changes size), the current amount of
padding, if any, is used as the new boundary.  This prevents a jarring
translation from occuring when the System UI changes size.

BUG=621633

Review-Url: https://codereview.chromium.org/2375113003
Cr-Commit-Position: refs/heads/master@{#423004}