Add xcompmgr to install-build-deps.sh

xcompmgr is a simple compositor manager to work with Xvfb.
It will be started by testing/xvfb.py in addition to Xvfb and
openbox, to make use of --enable-transparent-visuals in unit tests.

BUG=614512

R=danakj@chromium.org, dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2008313002
Cr-Commit-Position: refs/heads/master@{#395899}

build/install-build-deps.sh

@@ -109,7 +109,7 @@ dev_list="bison cdbs curl dpkg-dev elfutils devscripts fakeroot
           libwww-perl libxslt1-dev libxss-dev libxt-dev libxtst-dev openbox
           patch perl pkg-config python python-cherrypy3 python-crypto
           python-dev python-numpy python-opencv python-openssl python-psutil
-          python-yaml rpm ruby subversion ttf-dejavu-core wdiff zip
+          python-yaml rpm ruby subversion ttf-dejavu-core wdiff xcompmgr zip
           $chromeos_dev_list"
 
 # 64-bit systems need a minimum set of 32-bit compat packages for the pre-built