Use a forward-declare of views::View in views/border.h

There's currently a #include cycle between view.h and border.h

Also removes some redundant forward-declares in view.h

BUG=None

Review URL: https://codereview.chromium.org/93843002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238235 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238235