Remove DXSDK_DIR from search paths.

Chrome now builds ANGLE against the Windows 8 Platform SDK.

Move the include and lib search paths to common.gypi so that downstream projects can specify the location of the Platform SDK or alternatively use the DirectX SDK instead.
Review URL: https://codereview.appspot.com/7193052

git-svn-id: http://angleproject.googlecode.com/svn/trunk@1728 736b8ea6-26fd-11df-bfd4-992fa37f6226