Move common ChromotingView functionality into base class so that the code can

be shared between PepperView and X11View.

Have View create appropriate decoder based on encoding of the update stream.

Add state to decoder to keep track of whether Begin/End has been called in the
correct order.

Add unittests for new routines.

BUG=none
TEST=remoting unit tests

Review URL: http://codereview.chromium.org/3124005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@57206 0039d316-1c4b-4281-b951-d872f2087c98