Ken's answer complicates the subject a bit, I think.
Unity has a switch, -b, I think, that allows that. like unity -b http://mail.google.com
I'm not sure if it's really -b, so you better check with unity --help.
Finally I have figured out the problem and here is what I did :
Step 1 : Uninstall Google Chrome
Type the following in your terminal
sudo apt-get remove google-chrome-stable
Step 2 : Remove all chrome links
This was necessary because in my system, because Google Chrome icons were still present in spite of removing Chrome after Step 1.
sudo su
cd .local/share/applications
ls -l
I found that google-chrome-stable.desktop is still present here so I removed it by typing
rm -f -r google-chrome-stable.desktop
Step 3 : Reinstall Google Chrome
Download the latest google-chrome-stable_current_amd64.deb file (from here),
and install it using Ubuntu Software Center.
Step 4 : Post Installation settings (Most Important)
Navigate to /usr/share/applications directory using GUI, then drag and drop the Google Chrome icon into the docky.
Thats it...
Now you can launch Google Chrome from docky with just a single icon.
Best Answer
Press Alt+F2, enter
google-chrome --app=http://gmail.comthen pin the icon (right click it then click 'Pin to Dock') for the window that appears.chromium-browser --app=http://gmail.comis the equivalent command for Chromium.This will work for other websites too.