Fix vaapi_h264_decoder_unittest to use system.gyp:x11 instead of link line

We should never directly add X11 libraries to the link line as a given system
might need different cflags or ldflags. Depending on a gyp target is also
clearer than mucking with flags.

R=jochen

Review URL: https://codereview.chromium.org/198303003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256897 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256897