Include std vector (include what you use) in spdy_priority_forest.h

BUG=82098
R=jar@chromium.org

Review URL: https://codereview.chromium.org/104603003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242605 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242605