Hi Loic -
I'm not an expert on this topic, but it appears that google does not allow their pages to be displayed inside a frame. There is a very good reason for this - a hacking attack called "clickjacking".
I tried what you are doing while using a tool which records the HTTP requests and responses flying back and forth between the browser, Tableau Server, and Google. I see this response from Google:
HTTP/1.1 302 Found
Content-Type: text/html; charset=UTF-8
Set-Cookie: PREF=ID=c6e6213e59bad391:U=f60a0d5bd09d6409:FF=0:TM=1332165369:LM=1332165369:S=-fDc_lAJdg3-s_AM; expires=Wed, 19-Mar-2014 13:56:09 GMT; path=/; domain=.google.com
Date: Mon, 19 Mar 2012 13:56:09 GMT
X-XSS-Protection: 1; mode=block
The last option is extremely important - it is google saying "It is OK to display this content inside a frame *IF* the frame itself is being rendered on the same domain as the content" - Translated: "It's OK to show this google content inside a frame running on a webpage that is part of the google.com domain".
This is not the case for you -- your Tableau Server is not on the Google.fr domain.
It looks like you're not going to be able to do this based on the way google has set their stuff up.
Here's more reading, if you're interested:
Thanks for your answer! So the problem comes from this damned google.
Thanx for help,