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(e.g. scrolling, letterboxing the plugin, resizing the host) from
remoting.DesktopConnectedView into smaller single-purposed classes.

Summary of changes:
1. Moves bumpScrolling implementation into the remoting.BumpScroller.
2. Moves all viewport related implementation from remoting.DesktopConnectedView
   into remoting.DesktopViewport

BUG=457890

Review URL: https://codereview.chromium.org/918783002

Cr-Commit-Position: refs/heads/master@{#317707}