Unity Tweak tool fails to start with the error "com.canonical.unity.webapps schema is missing"

Install the package unity-webapps-service

Just a command line way to do jackyzy823's answer:

mkdir tweak;cd tweak
sudo apt-get download unity-webapps-service
ar xvf *
tar xvf data*
sudo mv usr/share/glib-2.0/schemas/com.canonical.unity.webapps.gschema.xml /usr/share/glib-2.0/schemas/
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
cd ..; sudo rm -Rf tweak

A tricky way for this,because i do not want to download so many dependencies for unity-webapps-service.

extract the file com.canonical.unity.webapps.gschema.xml (you can see it in http://packages.ubuntu.com/trusty/amd64/unity-webapps-service/filelist) in package unity-webapps-service,put this file to /usr/share/glib-2.0/schemas/ and finally sudo glib-compile-schemas /usr/share/glib-2.0/schemas/ and it works.