Remove IPC::Message::PRIORITY_NORMAL from cc_messages_perftest.

This ctor was removed in https://codereview.chromium.org/35643005/
but an unrelated bug was causing content tests to not be compiled so
this reference was not updated.

BUG=None
R=bbudge@chromium.org, danakj@chromium.org
TBR=piman@chromium.org

Review URL: https://codereview.chromium.org/48113005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231386 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231386