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Removing the include of Xlib.h makes selection_requestor.h more easily
testable. In particular, the definition of None in X.h conflicts with
that in gtest-type-util.h

This CL also adds XAtom to x11_types.h so that it does not need to be forward declared everywhere.

BUG=None
TEST=None
R=derat,danakj
TBR=dcheng (for trivial change to clipboard_aurax11.cc)

Review URL: https://codereview.chromium.org/392153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283740 0039d316-1c4b-4281-b951-d872f2087c98